Pulled in previous JavaScript grammar improvements into TypeScript grammar.

[aleppard]

TypeScript is a (typed) superset of the JavaScript language and there have been many improvements to the JavaScript grammar that haven't made it into the TypeScript grammar. This PR pulls them in, and adds a couple of minor TypeScript only improvements.

Improvements to the TypeScript grammar from the existing JavaScript grammar include:

• Improved import and export support.
• Addition of power (**, **=) and coalescing (??, ??=) operators.
• Improved support for async.
• Support for using underscore to break up long numbers.
• Support for big numbers.
• Better support for unicode characters.
• Support for class expressions.
• Member chain expressions.
• ... and more

The TypeScript specific improvements are:

• Fixed arbitrary nested generic types, e.g. Promise<Map<string, Set<string>>> now works.
• Added the 'null' type.
• The 'get' and 'set' identifiers are not reserved and can be used to create functions - just like in JavaScript grammar.
• catch clauses can now be typed, e.g. catch (error: MyError) {

I've copied a couple of the JavaScript examples to the TypeScript example directory to help test the improvements. It would be nice if the TypeScript tests made sure we could parse all of the JavaScript examples. However without copying all the files, I wasn't sure how to do that (and whether the intention was to deliberately decouple each language).

To review it might make sense to look at the diff TypeScriptParser.g4 with JavaScriptParser.g4 (ditto for lexers) as most of the changes are just existing grammar.

There is still more work to be done to bring Typescript up to date.