refactor: improve locale detection

refactor: improve locale detection

ECMA-402 support in Node.js should be reliable now, as it's building
with full-icu by default since Node.js 13. So I made it the first
choice.

Actually, I'm not even sure if the remaining variables are still useful.
But if they are used, here are my modifications:

- As for POSIX environment variables, I chose `LC_MESSAGES` over
`LC_CTYPE`, as it better reflects the purpose of the usage (we are using
the detection result to show prompt messages).
- I removed `LANGSPEC` because I can't find any documentation of it except
for one single mention in Wikipedia. I can't find any real usage of it
on my Windows machine.
- As we no longer use `LC_CTYPE`, it seems that `C` is no longer a
possible value for `shellLocale`, so I removed the fallback check logic.

If no more bug reports of Windows users are received in the coming
months, I think we can remove the TODO comment entirely.