Deprecate `[cp]` and `[.]`

[Aloso]

Status Quo

[codepoint] and [.] are special snowflakes: They can't contain other character classes, i.e. [. 'x'] is illegal.

Solution

Deprecate [codepoint] and [.].

• Instead of [codepoint], introduce a built-in variable Codepoint.
• Add aliases: Codepoint == C, Grapheme == G
• [.] can be replaced with ![n].
  • No built-in variable is added for the dot, since we don't want to produce . in the output; it doesn't work the same across regex engines, and its behavior is affected by several modes/flags/options.
  • For the user, Codepoint/C is usually better anyway. If a user happens to want to match anything except for line breaks, it's better to be explicit with ![vert_space].

Deprecation schedule

• [x] Add the built-ins Codepoint, C, and G
• [x] Update the documentation to recommend these built-ins
• [x] Remove documentation for the old syntax
• [x] Emit a deprecation warning in the CLI and the playground when the old syntax is used
• [x] Remove [cp] and [codepoint] syntax
• [ ] Remove [.] syntax -- let's wait until there is a replacement, if the dot really desired