Update C# lexer: new keywords and numeric literal syntax improvements

[stakx]

Those happened in C# language versions 6 through 9. I've mostly tested these additions visually (using rackup). Tests (rake) all pass. Here's a quick summary with links to official documentation:

Added keywords

• and (pattern matching in C# 9)
• init (init only setters and records in C# 9)
• unmanaged (new generic type constraint in C# 7)
• nint (native sized integers in C# 9)
• nuint (native sized integers in C# 9)
• not (pattern matching in C# 9)
• notnull (nullable reference types in C# 8)
• #nullable (nullable reference types in C# 8)
• or (pattern matching in C# 9)
• record (records in C# 9)
• with (records in C# 9)
• when (exception filters in C# 6, pattern matching in C# 7)

Numeric literal improvements (documentation)

• add C# 7 binary literals, e.g. 0b0010
• add C# 7 digit separators in all numeric literals, e.g. 123_567_890
• add missing M type suffix designating decimal
• make the exponent sign optional

[dblessing]

@jneen I'm curious on your thoughts here. I always recall you saying that a lexer doesn't necessarily have to be syntactically correct. In the case of this regex is there any reason we shouldn't accept the extended regex here for correctness? Or could we, for instance, collapse some of this and says an _ at the beginning or end is OK, although syntactically incorrect for C#?

          [_0-9]
          ([.][_0-9]*)? # decimal
          (e[+-][_0-9]+)? # exponent
          [fldum]? # type

[dblessing]

Thanks for your contribution, @stakx. I'm extending one question to Jeanine, but otherwise things look good. Let's wait a bit to hear back and if we don't, I'm good with it as-is.

[stakx]

This PR is probably out of date, given that it was opened almost 17 months ago, and there are two new versions of the C# language. Is there still any interest in this at all?

[tancnle]

@stakx Sorry for the delay on this PR 🙏🏼 . We are still interested in getting this merge. Any thoughts on whether this is still relevant given the new versions 10 and 11? We can always iterate on future PR.

[stakx]

@tancnle, yes, this should still be relevant, even now that there are newer language versions. Those add a few more new contextual keywords (e.g. file, init, required, scoped), but like you said, those can always be added in another PR.

[tancnle]

Thank you for your response @stakx. In that case, I am fine with this MR being merged 👍🏼 🚀