Swift 5.8 and 5.9 updates

[johnfairh]

Swift lexer updates for Swift 5.8 (and probably 5.7).
First commit:

• Add long-term missing keywords autoreleasepool left right
• Add recent new keywordsany distributed and #unavailable #fileID #filePath directives
• Add 'property wrapper projection' syntax -- means non-numeric identifiers can begin with $ symbol
• Remove incredibly old C-style __-prefixed things

Second commit is about regular expression literals which have some quirks due to being retrofitted to the language including:

• Single-line REs can't begin/end with spaces
• Single-line REs can have # delimiters which work like non-slash delims in other languages
• REs don't have trailing flags
• Multi-line REs must have # delims and always support extended syntax meaning #-end-of-line comments

There are some corners I haven't captured including turning off extended syntax inside multi-line REs.

Final commit re-alphabetizes some keyword lists without trying to think too much about why the separate lists exist.

[johnfairh]

I guess I will just keep going here for Swift 5.9:

Simple things:

• new declaration keywords package macro consuming borrowing
• new keywords each consume discard
• Nasty special ~Copyable type name - this is explicitly not a generic syntax (ie. for ~Fooable) so it seems least bad to specialise this explicitly :/

Swift 5.9 macros mean that users can define their own expression/function-type keywords that begin with ‘#’. So I’ve taken out the special cases of built-in #things as we’ve done in the past for @things. This looks good, highlights any arguments properly and improves the existing #available to match @available:

The special case for start-of-line is preserved to keep the 'preprocessor-like' parts different.

[tancnle]

Great stuff. Thank you for the work here @johnfairh 🙇🏼 LGTM 🚀