Enable `allowJs` with `isolatedDeclarations`

Acknowledgement

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Comment

Today, an error is issued when allowJs is set alongside isolatedDeclarations - js and ts .d.ts emit use the same pipeline, so it really should work, provided the error generation logic isn't making too many syntax assumptions.

Mostly, unlocking this is going to involve looking through the isolatedDeclarations tests and copying them with their cast/annotation syntaxes swapped to jsdoc and filenames swapped to .js just to give confidence things are going to work OK (and that getEffectiveTypeNode is being used over a direct .type get in enough places in the error generator). There's a few other JS-specific error cases to look at (js's unique export/expression merges, for one) that may need some new errors, but the bulk of it is just ensuring the existing type node presence checking logic generalizes to jsdoc-sourced type nodes.

Start by reviewing the existing isolatedDeclarations tests and the error generator, especially uses of getEffectiveTypeNode versus direct type access. Duplicate relevant cases with JSDoc annotations and .js filenames, then investigate JavaScript-specific export and expression-merge cases. Done means allowJs with isolatedDeclarations no longer fails incorrectly and the expanded tests pass.