refactor: simplify hosts to directly assign tsModule.sys where possible

[agilgur5]

Summary

Simplify host files with public func = tsModules.sys.func where possible

• Technically a follow-up to test: add initial unit test suite #321 as I was trying to get this to work at the same time, but ran into some testing issues (see below) and then backlogged it for a while as such

Details

• no need to duplicate types this way, which can and have changed over time (some of the function args have more parameters etc now actually) -- it's always the same typings this way

• also reorganize host.ts to have similar categories of functions near each other, instead of a mix of functions wherever

  • similar to how I organized the tests for host.ts as well

• shrink the code a bit this way too

• add a comment about getDefaultLibFileName's confusing naming pointing to the TS issues about how this is an old mistake but changing it now would be breaking (LanguageServiceHost.getDefaultLibFileName is confusing microsoft/TypeScript#35318)

• this is also how the TS Wiki recommends setting up hosts

  • NOTE: because of how tsproxy works to support alternate TS implementations, this does require that tsModule exists at the time of instantiation, i.e. that setTypescriptModule has already been called
    • for host.ts, LanguageServiceHost is only instantiated after setTypescriptModule, but for diagnostics-format-host.ts, it is immediately instantiated (at the bottom of the file), hence why getCurrentDirectory can't just be assigned to tsModule.sys
      • there is a way to fix this, but the refactoring for that is more complex as it would require creating in index.ts and then passing it as an argument -- would want to refactor more at that point too, so leaving that out for now in this otherwise small, isolated refactor
  • for a different, but related reason, the host.trace tests have to mock console instead of just console.log, since trace would just be set to the old, unmocked console.log otherwise
    • as it's assigned directly to console.log now