hl.UI.chooseFile multiple select support

[nspitko]

This is the haxe side change for HaxeFoundation/hashlink#559

This moves a bit of code around, and adds a new function hl.UI.chooseFileMultiple, which is a version of chooseFile that is always a load, and returns a path plus array of files.

The return format was chosen largely due to how the windows API works, so I'm not married to it if someone wants a different return format. I also would prefer a @:structInit class here (especially in hl) but I wanted to maintain consistency with the existing code.

Relevant API docs: https://docs.microsoft.com/en-us/windows/win32/api/commdlg/ns-commdlg-openfilenamea

Essentially this function takes the result from the hl-side chooseFile wrapper and breaks it up into a consistent haxe return. The code is fairly straightforward; we need to dig out each string in the return value, looking for the end marker (two nulls). We then handle the special case for a single file by breaking it out to keep the haxe side API consistent.

If this commit is accepted before the hashlink pull, the function will still function correctly, but not allow multiple file selections in the ui dialogue (making it basically useless, but not crashy/dangerous)

[yuxiaomao]

I don't really like changes on existing HL side API (return type in this case), it might break some Haxe+HL combination.
Maybe have another UI function at HL side (ui_choose_files / ui_choose_file_multiple, with return either a Ref path or Dynobj path, and NativeArray<Bytes> for files) and hide single/multiple files implementation details in C code?