zip.unzip: use built-in ZipReader instead of unzip command

[DeeJayLSP]

Another approach like #131. Closes #130.

Code is mostly taken from Godot's own documentation tutorial for unzipping files.

Installing an editor this way works just fine, but likely needs to be tested in other platforms.

Despite looking like a lazy copy of unzip_to_path(), I think it's important to get rid of the unzip command's need.