Source maps support

[guybedford]

It would be worth properly wiring through source maps for debugging lines to work out property with bundling.

Strictly speaking, since we now support bundling multiple input files together, this is a harder problem than simply handling a single input file with processed host imports. We could potentially revert handling bundling of local imports in the resolver (ie disabling support for import './asdf.js'). Alternatively we would need to work on first-class support for multi-source source mappings.

Another alternative for keeping single file script debugging, if we can disable bundling local files per the above, then we could even get away without source maps support and ensuring all injections are minified into a single first-line. This way debug lines would always be correct. It's not perfect, but it's a good win from the bundle reduction and can be followed-up with source maps support. Potentially even moving to multi-file.

Alternatively we can just implement multi-file source mappings, with a proper serialization scheme.

Between the above, I hope we can work on this debugging experience in some form at least.