Support optional props in TypeScript lexer

[pyrmont]

TypeScript allows a user to mark a property as an optional when defining an interface by appending a question mark to the property name. Rouge's TypeScript lexer doesn't currently support this because the lexer merely uses the states defined in the JavaScript lexer.

This PR prepends a rule to the statement state that will match a property name ending with a ?. This will allow matches outside of the interface definition but as syntax checking is a non-goal of Rouge this is not considered to be a problem.

The TypeScript visual sample is updated to include an example.

This fixes #1391.

[pyrmont]

@aaronadamsTO I think this fixes the problem you identified in #1391.

One thing I did note was that there seems to be a difference in how the question mark should be lexed. GitHub's syntax highlighter highlights the question mark differently to the prop name whereas the highlighting in the official TS docs doesn't.

Do you have an opinion on this? I'm currently following the TS docs but perhaps it would make more sense to be consistent with GitHub.

[aaronadamsCA]

@pyrmont Looking around at other lexers, there seems to be consistency in treating ? and : the same, as keywords, separate from the property name.

That said, I don't really have a strong opinion here! Either outcome seems fine to me.

[pyrmont]

@aaronadamsTO It seems a little strange to me to tokenise ? as a Keyword. I was going to go with Punctuation. Would that be wrong, do you think?

[aaronadamsCA]

@pyrmont Makes sense to me. Sorry, I meant "keyword" in the TextMate grammar sense - I'm new to Jekyll!

[pyrmont]

@aaronadamsTO Got it! Will update the token and merge :)

[pyrmont]

@aaronadamsTO OK—got it merged! The next release of Rouge is scheduled to be v3.16.0. That will come out 11 February :) Thanks for the help!