Premake Extension to support Emscripten

This is a fork of https://github.com/TurkeyMan/premake-emscripten with a bunch of changes:

wasm32 and wasm64 architectures
.wasm target extension
gmake and gmake2 actions
emscripten system
extension preload for --os=emscripten command line option support
fix emcc toolset validation in gmake actions
support for ninja build action via premake-ninja

And the original:

VisualStudio support through vs-tool