fix(vim/#2349): Crash on :enew

[bryphe]

Issue: :enew would crash the editor

Defect: We were crashing when trying to infer the language type from the filename - in this code:

oni2/src/Exthost/LanguageInfo.re

Line 136 in d3b6eb5

let fileName = Path.filename(fp);

The Rench.Path.filename (which uses Fpath under the hood) - is not robust against an empty filename.

Fix: Don't crash in that case - if it's an empty file name, just use the default ("plaintext") filetype. Add a regression test so we can ensure this doesn't break again in the future.

Longer term, I'd like to look at moving towards a strongly-typed solution for file path / name management - would like to integrate: https://github.com/facebookexperimental/reason-native/tree/master/src/fp

Fixes #2349