Allow language tests to be used by other Language extensions

[dplain90]

TS and JS Grammar Extension version: 0.0.49 / latest

As a VSCode Programming Language Extension creator I want to test my tmLanguage file using the TypeScript-TmLanguage test suite to ensure my extension supports all future and current TS language use-cases.

I've been building an extension that provides language support for jest spec.tsx/spec.ts files. Since I'm extending from TypeScript-TmLanguage, I'd like to use the snapshot tests in this repo to ensure any TypeScript use-cases are covered.

I made a PR, #713 , to lay the groundwork for that by abstracting the snapshot building process into a class that you can pass general configurations to.

It'd be nice if the testing build helpers and snapshot baselines lived in its own package and could be imported, however not sure the implications of that for ya'll...

Any thoughts?