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

• Add .gjs extension re-writing to addon.declarations when a project might be using gjs + gjs.d.ts embroider-build/embroider#2583

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