Support the `v` flag

[slevithan]

ES2024's v flag (unicodeSets, an upgrade to flag u) has been supported since Node.js 20 and 2023-era browsers. Currently, regexp-tree cannot parse the v flag, and as far as I know it can't parse any v-only syntax (nested character classes, set subtraction/intersection, properties of strings, \q{…}, etc.).

Using v is a best practice, so not supporting it makes it much harder to use regexp-tree in tools that want to offer support for modern JS regexes.