Support group with Perl flag setting and non-capturing group

[elton2048]

I see there is basic support for group but no some more complex features like Perl flag for named group and non-capturing group. It would be good if there is support on that.

I took a look of the code and seems there is no extra attributes for Expr (expression) since that is union(enum). Perhaps it would be a good idea to have new struct Group out such that there could be different attributes like name and captured bool flag to determine the type of that, for further expression handling?

Reference:

• https://github.com/golang/go/blob/3d33437c450aa74014ea1d41cd986b6ee6266984/src/regexp/syntax/parse.go#L1140 from Go lib
• https://github.com/alexnask/ctregex.zig/blob/5d5ac61c87ee71b6eb6f59ff9427ed2416398020/ctregex.zig#L597 from another Zig lib, unfortunately it seems not maintained anymore
• https://github.com/rust-lang/regex/blob/ab88aa5c6824ebe7c4b4c72fe5191681783b3a68/regex-syntax/src/ast/parse.rs#L1275 from Rust lib