Support import map

[CodeWitchBella]

My app uses import_map.json and there is no option in leaf to use this. I see that leaf uses Deno.emit which allows you to specify import map. Which means that this could be implemented by adding something like emitOptions to CompileOptions and spreading that into second argument of Deno.emit.

Does this look like something you would like to support in leaft? If yes, then I could send PR.

Another option would be to parse this from flags which would mirror how deno compile works.