refactor: combine two context files into one

[agilgur5]

Summary

Merge the two context files into a single file as they're intended to be as similar as possible. They're also not meant to be more extensible to create other contexts (unlike say, the cache interfaces etc), these are the only two contexts intended.

IMO, the slight, small distinction between them would've been easier to understand/catch on to if I saw them together like this than in separate files.

Details

• they're virtually identical, so combine them instead of keeping them separate

  • changes to one should probably be made to both
  • still a < 100 LoC file

• refactor out _.isFunction with a simple getText function instead

  • checks the opposite
  • one more lodash removal! (follow-up to #328)

• add docstrings about when to use the two contexts

Future Work

• I'd like to do something similar with the diagnostics files and the tsconfig files, as they're also only used in the same context and not in other contexts. Planning to add more unit tests first though

[agilgur5]

Very likely that ConsoleContext will be removed per my realization in https://github.com/ezolenko/rollup-plugin-typescript2/pull/345#issuecomment-1200277838, but this PR will also make it easier to remove it as all the code is now in this one file and all the imports are from this one file