consume sourcemap directive when parsing source code

according to our conversation yesterday, if the sourcemap is base64 embeded in the source code, we should do this:
- extract it before calling esprima (for perf reasons)
- digest the sourcemap as the input sourcemap

Locate the source-parsing entry point and the call to esprima. Trace how embedded base64 sourcemaps are represented, then ensure the directive is consumed before parsing and the sourcemap is digested as the input sourcemap; add coverage for embedded sourcemap input if the existing test structure provides a suitable place.

Written by the indexing model from the issue text.