Repro: declarations for gjs are not re-written when generating types

[NullVoxPopuli]

Note:

• the "dts" for a gjs file is ".gjs.d.ts"
• doing this requires setting allowArbitraryExtensions: true in the tsconfig.json -- which is probably something we can upstream in to @ember/library-tsconfig as a non-breaking change since it's an enabling of something that would have been prevented before

Once

• https://github.com/embroider-build/embroider/pull/2583

is merged and released, this PR should start passing after updating to that new release