Problem:
Version parsing makes some assumptions about the `nvim -v` output that
could be more robust. For example,
- "NVIM" may change to "Nvim": neovim/neovim@98ba65b
- "v" is not guaranteed to precede the version

Solution:
- match case-insensitive "NVIM"
- match "v?" instead of "v"
- Note: `--api-info` is not used because it is a big payload (thus
  requires streamed decoding, thus more complex code).

Problem:
parseVersion() does not handle invalid version strings. For example if
findNvim() passes a partial or invalid  version string, or undefined.

Solution:
- If compareVersions() arg 1 is invalid, throw an error.
- If compareVersions() arg 2 is invalid, treat it as "older"/"less".