(fix): parse tsconfig extends, trailing commas, and comments

[agilgur5]

• use ts.readConfigFile to properly parse a commas and comments from
  tsconfig instead of just trying to readJSON

• use ts.parseJsonConfigFileContent to properly parse extends and
  get recursive compilerOptions

• add tests for all of the above use cases

  • add them to build-withTsconfig which changes declarationDir;
    should only work if TSDX properly parses the tsconfig

NOTE: this is only necessary for internal, TSDX-specific parsing of
tsconfig.json. rollup-plugin-typescript2 already handles these options,
but internal options like esModuleInterop etc are also used & parsed
independently of rpts2

Fixes #483 with its own suggestion, eventually getting to facebook/create-react-app#7248 as the solution EDIT: that only solved commas and comments, extends needed more work, see below comments.
Also fixes #484 as it's the same parser that does both. Still need to add tests for this case, thinking of just having all test fixtures extend a base fixture and then override it?

This is built on top of #436 , please merge that first.

[jaredpalmer]

Fix conflicts

[agilgur5]

Done.

[agilgur5]

Welp, so since I merged #468 , I switched to use that fixture, which actually does something different than the default, i.e. sets declarationDir. So if declarationDir works properly, then it means this works... except it didn't 😕

Turns out ts.readConfigFile will handle commas and comments and such, but not extends... 😕 😕 It just returns the JSON with an extends.

Seems like the same issue encountered in kulshekhar/ts-jest#245 (comment) , the solution to which is non-trivial... welppppp. investigating alternatives right now

[agilgur5]

Ok, some links I went through to figure this out:

• [Improvement] Let typescript load the config file, instead of tsconfig package kulshekhar/ts-jest#245 (comment)
  • the PR for which replaced the older TypeStrong/tsconfig
    • which has this issue: Support tsconfig inheritance TypeStrong/tsconfig#21 (comment)
      • which pointed to https://stackoverflow.com/a/53898219/1046439
• also looked at various things in CRA like Respect tsconfig.json extends when validating config facebook/create-react-app#5537
• eventually searched in rpts2 and found this line which is much simpler than all of the above

So ended up using that last one

[agilgur5]

LGTM. I reduced the necessary changes for extends to just a few lines, so there wasn't much to review.

I'm still a bit skeptical of the './' for ts.parseJsonConfigFileContent's third argument, basePath, but this passed all of my manual tests too. I'm most concerned about usage with custom tsconfig path #436 especially since we don't have tests for that. Will have to add some soon.