safe identifier characters should include all Unicode alphanumerics

The documentation for safe identifiers says "alphanumeric" characters are allowed, but the implementation supports only ASCII letters and numbers. Unless there are security issues with Unicode characters, they should be supported.

Motivation: documentation sites that want to use language identifiers as (part of) fragments for easy navigation, e.g. https://pkg.go.dev.

The change I'm suggesting would be from
```
var onlyAlphanumericsOrHyphenPattern = regexp.MustCompile(`^[-_a-zA-Z0-9]*$`)
```
(https://github.com/google/safehtml/blob/v0.0.2/identifier.go#L49)
to
```
var onlyAlphanumericsOrHyphenPattern = regexp.MustCompile(`^[-_\pL\pN]*$`)
```