fix: Improve file ignoring

[andyrooger]

Addresses a few problems with ignoring files:

• File ignore checking is async. There's no guarantee it's happened before we need to know the answer.
  • In my case I was seeing hundreds of thousands of lint errors from ignored files (in node_modules), though some did get ignored
  • I've made as much of the ignore checking synchronous as I can. ESLint does the check synchronously too so as far as I can tell, it's just receiving the result that happens async. Results are not perfect but in my case I see ~3000 unwanted lint errors rather than ~250000.
  • Only a partial fix, but I don't think there's more that can be done unless we choose to access the deprecated CLIEngine directly, or TS supports async plugins.
• Hard-coded ignores had the opposite effect
  • They were just forcing those files to not be checked against eslint ignores - effectively forcing them to be linted.
  • I've made sure the hard-coded ignores happen synchronously, and that they actually cause the files to be ignored.
• Windows paths weren't well supported
  • These were coming through with both forward and backslashes in Visual Studio Enterprise for me. They're also case insensitive, though the cases seemed to be consistent for me.
  • I've made the hardcoded ignores account for this, and normalized the paths before caching ignore results.