API: "find Nvim" function

[justinmk]

UIs and other Nvim applications often search for Nvim in the system/environment, or else they require users to specify an exact path.

• similar usecase is firenvim's get_progpath() though that obviously requires Nvim already.

pynvim could save a lot of hassle by providing a function like:

find_nvim(min_version: string)

with this behavior:

1. finds nvim in the current environment at a given minimum version
2. If nvim not found at given version, return an error that mentions the version that was found
3. return a usable path (like aforementioned firenvim get_progpath logic)
   • Actually this could be in nvim itself. Maybe v:progpath should be smarter?

see also: https://github.com/neovim/node-client/issues/146