refactor: prefer native methods to lodash where possible

[agilgur5]

Summary

Replace most (not all) lodash methods with native methods

Details

• _.endsWith -> String.endsWith

• _.concat -> Array.concat

• _.each -> Array.forEach

• _.filter -> Array.filter

• _.map -> Array.map

• _.some -> Array.some

• _.has -> key in Object

• _.defaults -> Object.assign

• _.get -> ?. and ?? (optional chaining and nullish coalescing)

• refactor: replace fairly complicated expandIncludeWithDirs func to just use a few simple forEachs

  • not as FP anymore, more imperative, but much simpler to read IMO

• refactor: add a getDiagnostics helper to DRY up some code

  • also aids readability IMO

• a few places are still using lodash, but this paves the way toward removing it or replacing it with much smaller individual deps

  • _.compact still used because Array.filter heavily complicates the type-checking currently
  • _.isFunction still used because while it's a one-liner natively, need to import a function in several places
    • also the package lodash.isFunction is lodash v3 and quite different from the v4 implementation, so couldn't replace with it unfortunately
  • _.merge is a deep merge, so there's no native version of this
    • but we may remove deep merges entirely in the future (as tsconfig doesn't quite perform a deep merge, c.f. Shallow merge in tsconfigOverride where TS does so -- match tsconfig extends #86 (comment)), or could replace this with a smaller lodash.merge package or similar

• see also https://github.com/you-dont-need/You-Dont-Need-Lodash-Underscore

Review Notes

Unit tests still pass, so other than looking equivalent and typing equivalently, usage still works equivalently (caveat though: tscache and index aren't yet tested)