Support Win32 style PATH

[Bottersnike]

Windows uses ; as the separator in PATH as opposed to : on unix-like systems.

[Bottersnike]

As a side-note, Windows doesn't require the cwd to be on PATH, as it implicitly searches there first. It may be preferable to omit path modification entirely on windows. Likewise .exe is unnecessary on windows, though may be desired anyway?

Edit: I just had a check, and lute relies on libuv for process spawning. This bypasses a lot of the normal windows mechanisms. From what I can tell, libuv's search order is mostly analogous to how windows normally searches for paths, and notably does first search the cwd, and will also implicitly add .com or .exe if omitted from the name. I think based on that it's probably preferable to omit adding .exe or modifying PATH entirely, but it's your call which is preferred. I checked and / is coerced into \\ so no issues elsewhere regarding that