Sourcemap: Expose mappings field

[michaeldiamant]

Subject of the issue

#3726 unintentionally introduced a spec mismatch. The deviation precludes sourcemap consumers from using standard sourcemap tooling.

Expected behaviour

• The returned sourcemap includes a mappings key https://sourcemaps.info/spec.html.
• The mapping key is marked as deprecated, but continues to be populated.

Actual behaviour

The returned sourcemap provides a mapping instead of a mappings key.