Support trailing commas in `<catchPart>`

For consistency, we could support a trailing comma in a ``:

```ebnf
::= 'catch' '(' (`,' )? ','? ')'
```

This proposal is a supplement to https://github.com/dart-lang/language/issues/1402 and https://github.com/dart-lang/language/issues/2430 where trailing commas are proposed in other syntactic positions.

NB: If we do support the trailing commas than the CFE should be adjusted to accept the trailing comma with a single identifier, and the analyzer should be adjusted to accept both. Conversely if we do not adopt this proposal then the CFE should be adjusted to reject the case with two identifiers. So we do need to do something in order to obtain a consistent treatment in both cases.

Start with the proposed EBNF and the related issues 1402 and 2430 to understand the intended consistency. Then inspect how the CFE and analyzer handle a single identifier versus two identifiers in . Done means both components consistently accept or reject the trailing-comma cases.

Written by the indexing model from the issue text.