No support for \p{unicode char class} that is supported by the Go stdlib regex package

Extract from https://pkg.go.dev/regexp/syntax

```
\pN            Unicode character class (one-letter name)
\p{Greek}      Unicode character class
\PN            negated Unicode character class (one-letter name)
\P{Greek}      negated Unicode character class
```

I imagine this could be implemented in RE2 compat mode.