To improve consistency with other clocks and readability of the code,
Update the default values of the variables in ImmediateClock.

e.g: TestClock initialization

```swift
    public var minimumResolution: Duration = .zero
    public private(set) var now: Instant

    private let lock = NSRecursiveLock()
    private var suspensions:
      [(
        id: UUID,
        deadline: Instant,
        continuation: AsyncThrowingStream<Never, Error>.Continuation
      )] = []

    public init(now: Instant = .init()) {
      self.now = now
    }
   ```

…28)

There is no need for 'offset' to be a var.

A few APIs were `public` when they shouldn't be. They aren't primary
APIs, and it is unlikely that folks rely on them. So though this is a
breaking change, it shouldn't affect folks, and there is an alternative
API they can rely on instead.

`Shims.swift` is no longer needed since `sleep(for:tolerance:)` is now a
[native
function](https://developer.apple.com/documentation/swift/clock/sleep(for:tolerance:)).

We can drop Swift <5.9 now that the App Store requires Xcode 15 for
submissions, and we can add 6.0 language mode to keep our concurrency
story in check.

Xcode 16 added support for the [EditorConfig
standard](https://editorconfig.org/)[^1]. This allows a project/repo to
specify basic formatting rules so the editor can behave correctly.[^2]

> [!NOTE]
> You may need to quit and relaunch Xcode for it to pick up the
`.editorconfig` file after switching to a branch where it's present.[^3]

The added `.editorconfig` file contains:

```
# editorconfig.org
root = true

[*]
indent_style = space
indent_size = 2
trim_trailing_whitespace = true
insert_final_newline = true
```

- `root = true`: Specifies that this is the top-most .editorconfig file.
The file search will stop here.
- `indent_style = space`: Uses soft tabs (spaces) for indentation
instead of hard tabs.
- `indent_size = 2`: Sets the indentation to 2 columns.
- `trim_trailing_whitespace = true`: Removes any whitespace characters
preceding newline characters.
- `insert_final_newline = true`: Ensures the file ends with a newline
when saving.

These settings apply to all files in the project (`[*]`).

**This change make much easier the process of switch between projects
that use 2-space and 4-space indentation (what is quite common in your
community).**

[^1]: [Xcode 16 Beta 2 Release Notes–Source Editor New
Features](https://arc.net/l/quote/zuzqnfeq)
[^2]: Inspired by: [Add an EditorConfig
file](swiftlang/swift-syntax#2714)
[^3]: [Xcode 16 Beta 2 Release Notes–Source Editor Known
Issues](https://arc.net/l/quote/olmnhsqo)

Makes it so `TestClock` and `UnimplementedClock` both report failures to
the new Swift Testing framework.

---------

Co-authored-by: Brandon Williams <[email protected]>