std(os/windows): Add GetTempPathW and GetTempPath

[abhinav]

This change adds a kernel32.GetTempPathW binding to GetTempPathW.
This function returns the system path for temporary directories
(e.g. "C:\Temp").

The accompanying windows.GetTempPath function produces a UTF-8 string,
exposing an API with a similar pattern to GetCurrentDirectory.

[squeek502]

Just a heads up: this is unlikely to be merged unless there's a use-case within the standard library or the Zig compiler source code.

Relevant issues:

• Prefer depending on NtDll rather than kernel32 or other higher level DLLs #1840
• reduce windows API bindings in the standard library to the minimum required for the standard library's own abstractions to work #4426

Note also that this does not have to be part of the standard library for you to be able to use it. This:

pub extern "kernel32" fn GetTempPathW(
    nBufferLength: u32,
    lpBuffer: [*]u16,
) callconv(WINAPI) u32;

can go in your project's source and it'll work just fine.

[abhinav]

Note also that this does not have to be part of the standard library for you to be able to use it.

Ah, thanks for verifying. I suspected this but didn't have an easy way to verify it.

Yeah, it makes sense if the Windows bindings are limited to things std needs.
I can imagine a need for this and a unix analog in the future, but this PR can be closed meanwhile.