Make catch clause variables optional

[The proposal](https://michaelficarra.github.io/optional-catch-binding-proposal/) by @michaelficarra makes the binding identifier in catch clauses optional.

For example:

```ts
try {
    JSON.parse("...");
}
catch {
    // Don't care, the error message is completely useless,
    // but we need to handle it.
}
```

This proposal reached stage 3 immediately upon presentation.