fix(util): Path.parent now works on windows (#1168)

[xudyang1]

On Windows, both forward slash / and backslash \\ work as the path separator. Linux and MacOS can have backslash as a valid filename character.

Unit tests are also provided for each platform because Path.parent depends on the local variable sep which depends on jit.os.

Fixes #1168

[L3MON4D3]

Hey, thanks for tackling that :)

This looks good (differentiating on definition-level, not at runtime is cool :D), and I'd like to propose two improvements/changes:

1. parent should retain the previous behaviour of removing the trailing sep from the path, that fits with the remainder of the current code (join always appends sep, for example)
2. Regarding testing: I like that you added tests for both platforms, but since these tests can be run on either platform, it would be cool to also do this. For example, you could (in the definition of parent) allow some global variable that is unlikely to be set during normal operations, maybe ___LUASNIP_TEST_SEP_OVERRIDE, to override what we detect via sep, and then set that to / or \ depending on which test is currently running

[xudyang1]

Hey, thanks for tackling that :)

This looks good (differentiating on definition-level, not at runtime is cool :D), and I'd like to propose two improvements/changes:

1. parent should retain the previous behaviour of removing the trailing sep from the path, that fits with the remainder of the current code (join always appends sep, for example)
2. Regarding testing: I like that you added tests for both platforms, but since these tests can be run on either platform, it would be cool to also do this. For example, you could (in the definition of parent) allow some global variable that is unlikely to be set during normal operations, maybe ___LUASNIP_TEST_SEP_OVERRIDE, to override what we detect via sep, and then set that to / or \ depending on which test is currently running

If I understand that correctly:

1. the previous behavior of parent returns parent directory without trailing sep for files only, but for directories, for example /home/user/, it returns nil but not /home. To retain the previous behavior, require("luasnip.util.path").parent("/somefile.txt") should return "" (empty string). Although it looks weird, this is a valid parent because empty string in lua is a truthy value. I will also add some docs to the parent function to clarity the above behaviors.
2. Instead of making tests platform dependent, we should make them path separator dependent so that the tests can even run on other platforms that use either forward slash or backslash as path separator.

[L3MON4D3]

Nice, this looks great!
Thanks for adding the disclaimer on C: for windows, I doubt there will ever be a case where a snippet-repo lives in a root-directory, but this should find it :) (and if that ever happens, maybe we can just switch over to the fs-module built into neovim :) )

Thank you!!