Replace the standard Lua include path entirely on Windows

The default Lua include path is encoded with the local charset rather
than UTF-8. This causes problems when the working directory includes
non-latin-1 characters which are valid in the local charset, as Aegisub
expects the string to be UTF-8. As the default path isn't very useful on
Windows (on other platforms it could potentially include things like
luarocks-installed modules), just clear it.

Closes #1676.