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