Consistent `@command args...` syntax for all special character strings

[gadenbuie]

I like the @news pkg {optional_search} syntax. It does make me wonder if, rather than curly quotes with{dplyr}, we ought to support @pkg dplyr. This would align with the tidy style convention of no special formatting for package names, bring this syntax a little closer to other btw shorthands, and bring the syntax a little closer to e.g. Positron Assistant participants. (This argument could also go for @help topic instead of ?topic, etc, so that folks would only need to remember the @ keyword to use shorthands with btw. Maybe this should be spun off into a more general issue.)

Originally posted by @simonpcouch in #64 (review)

btw("{dplyr}") # now
btw("@pkg dplyr") # and also

btw("?dplyr::across") # now
btw("@help dplyr across") # and also
btw("@help dplyr::across") # also also

# etc...