Both `up` and `down` only show 1 source line when called, I think it
would be more useful if there was a configurable way to provide more
context.

This adds a `show_src_lines_frame` configuration option (which defaults
to `1`) that determines how many source lines get shown for frame operations `up`, `down`, and `set` (frame).

Since ruby 3.4, not all cases match BLOCK_LABL_REGEXP anymore
This fixes for those cases.

Just that - normalizing various formatting, code blocks, fixing some
grammar and typos, and the like.

Entering an empty newline repeats the last command, such as `continue`
or `list`.

This configuration disable this feature.

ruby#1098

```
$ ruby -Ilib exe/rdbg rescue-test.rb
[1, 7] in rescue-test.rb
=>   1| 1.times do
     2|   begin
     3|     raise
     4|   rescue
     5|     p 1
     6|   end
     7| end
=>#0    <main> at rescue-test.rb:1
(rdbg) s    # step command
[1, 7] in rescue-test.rb
     1| 1.times do
     2|   begin
=>   3|     raise
     4|   rescue
     5|     p 1
     6|   end
     7| end
=>#0    block in <main> at rescue-test.rb:3
  ruby#1    Integer#times at <internal:numeric>:257
  # and 1 frames (use `bt' command for all frames)
(rdbg) s    # step command
/home/mame/work/debug/lib/debug/thread_client.rb:85:in 'DEBUGGER__::ThreadClient#default_frame_formatter': undefined method '+' for nil (NoMethodError)

          "#{colorize_blue("block")}#{args_str} in #{colorize_blue(block_loc + level)}"
                                                                             ^
        from /home/mame/work/debug/lib/debug/thread_client.rb:755:in 'Method#call'
        from /home/mame/work/debug/lib/debug/thread_client.rb:755:in 'DEBUGGER__::ThreadClient#frame_str'
        from /home/mame/work/debug/lib/debug/thread_client.rb:742:in 'block in DEBUGGER__::ThreadClient#show_frames'
        from <internal:numeric>:257:in 'Integer#times'
        from /home/mame/work/debug/lib/debug/thread_client.rb:739:in 'DEBUGGER__::ThreadClient#show_frames'
        from /home/mame/work/debug/lib/debug/thread_client.rb:304:in 'DEBUGGER__::ThreadClient#suspend'
        from /home/mame/work/debug/lib/debug/thread_client.rb:358:in 'block in DEBUGGER__::ThreadClient#step_tp'
        from rescue-test.rb:5:in 'block in <main>'
        from <internal:numeric>:257:in 'Integer#times'
        from rescue-test.rb:1:in '<main>'
rescue-test.rb:3:in 'block in <main>': unhandled exception
        from <internal:numeric>:257:in 'Integer#times'
        from rescue-test.rb:1:in '<main>'
```

This is because `rb_debug_inspector_backtrace_locations` returned a
modified backtrace, which skips rescue/ensure frames, but
`rb_debug_inspector_frame_XXX_get`'s index is considered for a raw
backtrace, which includes rescue/ensure frames.

The problem wil be fixed by ruby/ruby#13510.
However, now the backtrace includes rescue/ensure frames, so some tests
in debug gem fails. This fixes the test failures.

In Ruby 3.5, `Kernel#caller_location` will not include `<internal:xxx>`
frames. ruby/ruby#13238

To skip the internal frames, explicitly check if `TracePoint#path` is a
internal path or not.

When running the `eval` command (or its alias, `call`) without an
argument, you get the following error:

```
[REPL ERROR] #<NameError: undefined local variable or method `cmd' for #<DEBUGGER__::Session:0x0000000120a0fa20>>
  debug-1.10.0/lib/debug/session.rb:934:in `block in register_default_command'
  debug-1.10.0/lib/debug/session.rb:1165:in `process_command'
  ...
```

...instead of this hint:

```
To evaluate the variable `eval`, use `pp eval` instead.
```

I believe the regression was introduced in a402e73.

To fix this issue, I was thinking of passing the command name alongside
the argument in the `process_line` method but I wasn't sure this was
worth it just for this hint. So I removed the hint altogether instead.

This commit implements XDG directory support for this gem's history file
in accordance with the rules outlined in ruby#1031:

> For the history file:
>
> 1. prefer `~/.rdbg_history` if present,
> 2. else, `$XDG_DATA_HOME/rdbg/history` if `$XDG_DATA_HOME` is set¹
>
> ¹ There'd need to be a check for this file path. If it exists, great!
> If not, create the path `$XDG_DATA_HOME/rdbg` and touch
> `$XDG_DATA_HOME/rdbg/history`.

See: ruby#1031 (comment)

This commit implements XDG directory support for this gem's `.rdbgrc`
configuration file in accordance with the rules outlined in ruby#1031:

> 1. prefer `~/.rdbgrc` if present,
> 2. else, `$XDG_CONFIG_HOME/rdbg/config` if `$XDG_CONFIG_HOME` is set
> and `$XDG_CONFIG_HOME/rdbg/config` is present,
> 3. else, no customized user configuration

See: ruby#1031 (comment)

* Set `CONFIG[:history_file]` as nil, and ignore it if it is nil.
* Respect `CONFIG[:history_file]` even if the path doesn't exists.
* Respect `~/.rdbg_history` if exists for compatibility.
* Use `XDG_STATE_HOME` if there is no configuration about history file.

The imemo mask has been `0xf` since Ruby 2.5 /
ccfe37884ab566336380d0f21e15321d6382da8f

This hasn't caused problems yet because the only
possible conflict is `imemo_type = 15`, but it is
not yet used.