`getCurrentDirectory` is marked as optional on LanguageServiceHost, but absence is an error

[weswigham]

getCurrentDirectory is optional on LanguageServiceHost (as it inherits from GetEffectiveTypeRootsHost, and it is optional there), but calling createLanguageService with a host which does not implement getCurrentDirectory throws TypeError: host.getCurrentDirectory is not a function.

[weswigham]

@mhegazy I don't think this is a breaking change. The change that made it optional was only comitted yesterday. Plus, we crash in createLanguageService if you actually don't provide it, so...

[mhegazy]

oh, i see. i thought the fix was to change GetEffectiveTypeRootsHost.getCurrentDirectory which is an overkill here.. you are correct. let's get this in then.