Fix sourcemaps for GJS

[NullVoxPopuli]

Using @embroider/[email protected], I noticed this message:

(!) Broken sourcemap
https://rollupjs.org/troubleshooting/#warning-sourcemap-is-likely-to-be-incorrect
Plugins that transform code (such as "rollup-gjs-plugin") should generate accompanying sourcemaps.

This PR fixes that

This PR enables viewing actual gts and gjs in the browser, as original source

and most importantly, breakpoints line up:

[NullVoxPopuli]

Superseded by #2166