Add cop that checks for valid sigil

[peterzhu2118]

Adds a new cop ValidSorbetSigil that checks that every Ruby file contains a valid Sorbet sigil. Has autocorrect features that adds typed: false to all files that do not have a sigil. I've tested this on core and it works as expected and causes no typecheck errors.

I've talked to @clarkdave and has given me permission to adapt from this gist: https://gist.github.com/clarkdave/85aca4e16f33fd52aceb6a0a29936e52.

[Morriar]

Thanks for indulging me!

[XrXr]

Instead of searching twice, why not search the first block of comment tokens?

something like: processed_source.tokens.take_while {|token| token.type == :tCOMMENT}.find { |token| /^ # typed: (\w+)/.match(token.text){|m| sigil = m[1] }; !!sigil }

This way you wouldn't have to search twice to find the location of the token when you want to report the error.

[paracycle]

Not sure what the rule for matching the comment is, but I think we can be a bit more flexible with it.

[paracycle]

Awesome!