Add metadata for source maps

[dominiccooney]

JavaScript embeds sourcemaps with magic comments like //# sourceMappingURL=/path/file.js.map. BinAST files will also need metadata like this.

BinAST may also need to amend the sourcemap format, because BinAST files do not have line and column numbers or, for that matter, even character-aligned data.

[sendilkumarn]

This means the sourcemap will be in a separate format & rules like .binmap
or can we just decode binast into js and use the sourcemap w.r.t js if that is easier & faster to do. (anyways it will be mostly used only in the development)

[Yoric]

I like @sendilkumarn's idea. If we can somehow generate a sourcemap from the decoded binast, we don't need to amend the sourcemap format. Can we?