Make trailing commas on binding/rest not an error in .d.ts files

[weswigham]

#22262 is pretty breaky, and the change is to be more spec compliant. To reduce how breaky the change is, we shouldn't report the error if it happens in a .d.ts file, as there will be no associated emit - this way older .d.ts files won't need to be updated just to remove trailing commas that we can actually handle just fine.

[unional]

Can we emit a warning instead? Silent is not a good solution either.

[weswigham]

Declaration files will never be executed and are just metadata (for the compiler itself) - I think we can silently be more permissive in them.

[unional]

But it would be nice to be able to detect them somehow so that typings authors can fix them easily.

Another way is to rely on ts-lint auto-fix to remove all trailing commas. :)

[weswigham]

Those tools can still find and remove trailing rest/bind commas; we just don't need to report an error in a .d.ts - it's unhelpful and unneeded.

[another-guy]

#23139 results in breaking code builds with error TS1013: A rest parameter or binding pattern may not have a trailing comma.

In my case, it happens in non *.d.ts code.

I agree with @unional, it should be a warning, unless bumping a major version is okay for such a small change.