Allow generating sourcemaps for Lua in the same format as JS

[Zorbn]

Lua tools (Local Lua Debugger, TypescriptToLua, etc) commonly use the same format for sourcemaps as JS does. This PR moves code specific to generating JS-style sourcemaps into jsSourcemap.ml then uses that code to implement sourcemaps for Lua.

[Simn]

I think the original plan was to have all the generic sourcemap functionality in sourcemaps.ml. I really like the separation from genjs.ml regardless though, and if lua sourcemaps are so similar it makes sense to handle it like that.

[Zorbn]

Lua and JavaScript sourcemaps are pretty much identical because they follow the Source Map Revision 3 spec. PHP is the language using sourcemaps.ml right now and I don't know much about PHP. If their sourcemaps also follow the same spec then sourcemaps.ml and jsSourcemap.ml could be combined. Of course then sourcemaps.ml would be pretty JavaScript centric since it would be supporting 3 types of sourcemaps based on a JavaScript spec. Maybe that's ok if there aren't any other types of sourcemaps we would need to support (I don't even know if other types of sourcemaps exist).

[Simn]

Thanks!

[RealyUniqueName]

There's no sourcemap concept in PHP world.
sourcemaps.ml uses JS sourcemap spec.