[Regular Expressions] Single opening bracket in charsets is scoped punctuation

### What happened?


A single opening `[` may appear unescaped in charsets and thus should not be scoped `punctuation.definition.set`.

```regex
[[]
```

It becomes punctuation not before valid nestedcharset is present.

```regex
[[:alpha:]]

or

[[A-Z]-[0-9]]
```
