Use LanguageService as solution for emit-less types and semantic diagnostics

[kryops]

This PR adds a useLanguageService option to the plugin. It uses the TypeScript LanguageService for compilation instead of the existing typescript.compileModule() functionality.

Pros

• Imports of emit-less types are stripped (Allow modular use of emit-less types #28)
• Diagnostics for semantic errors (Plugin doesn't output non-fatal errors. #43)

Cons

• Slower compilation
• More complex API

Unresolved issues

• Typings in node_modules/@types aren't discovered automatically with my current LanguageService implementation. As a workaround the names of all folders in node_modules/@types are set as compilerOptions.types if none are present already.
• I could not get the tests to run and thus didn't add any new tests. I got the following error message:

TypeError: Path must be a string. Received 6
    at assertPath (path.js:7:11)
    at Object.join (path.js:1211:7)
    at Object.<anonymous> (/home/michael/git/rollup-plugin-typescript/node_modules/buble/register.js:41:22)
    ...

Any feedback or help regarding the unresolved issues is greatly appreciated!

[tsbehlman]

This fixes #43 for me ❤️

[PaulBGD]

This is very cool, I'm definitely interested in integrating it into #91. Would you like to rebase the PR off the progress branch? Otherwise I can do it myself.

[kryops]

@PaulBGD I don't have that much spare time at the moment, might be able to do it on the weekend. Feel free to give it a try yourself if you can't wait 😉

As a basis for this PR I used the following documentation: https://github.com/Microsoft/TypeScript/wiki/Using-the-Compiler-API#incremental-build-support-using-the-language-services

[PaulBGD]

Thanks! I'll check it out :)

[PaulBGD]

So quick question, why do you only throw on fatal errors when it's the 1st run?

Also closing this since I integrated it with #91. Everything looks great.

[kryops]

I'm pretty sure that was to stop it from crashing when I'm in watch mode and introduce a TypeScript error. I want the initial build to fail when it has errors, but once the watch mode is running, I don't want to have to restart it every time I introduce an error

[PaulBGD]

That's actually a rollup-watch error, I'll see if I can get that fixed.