Add path separator constants and utility function.

[aznashwan]

Includes separate commit with minor docstring formatting changes

[lpil]

Hello! What's the motivator for this change? Thank you.

[aznashwan]

It's useful for people who find themselves needing to join paths for a different system (e.g. if you're on Unix and want to compose a Windows UNC share path to send to a remote Windows system).

It's also useful to know what path separator the current system you're running on is using if you need to manipulate paths directly.

Go has one FWIW: https://pkg.go.dev/path/filepath#pkg-constants
Go's is also defined separately on Windows and Unix systems.

[lpil]

In Gleam we don't like to have multiple ways to do the same thing. Given we have functions for both splitting and joining paths wouldn't this be duplication?

[aznashwan]

There is currently no way to join a Windows path with this package without relying on something else transparently translating the slashes into backslashes for you (which most Windows syscalls fortunately transparenty do, but it's better to be safe than sorry).

This package should at the very least be changed so that:

• it exposes separate join_windows() and join_unix() functions
• join() is switched to using the correct version of the above (currently, it only ever joins with slashes)

Both of the above points were beyond the scope of my original patch (#5) and I intended on adding them later with the help of these constants/function, but I got side-tracked with the issues with split() and is_absolute() on Windows and here we are...

I can make these separator constants/function private and convert this PR to include the above changes if you wish.

[lpil]

Duplicate APIs are not an option so it sounds like the solution is to make the join function use \ on Windows.