[FEA] Improve regexp testing with regexp-aware fuzzer

[andygrove]

Is your feature request related to a problem? Please describe.
The use of fuzzing has been very effective in finding unsupported edge cases in the regular expression support but I would now like to add an additional fuzzing technique where we aim to generate (mostly) valid regular expressions by using a fuzzer to generate a regexp AST made of valid constructs. This will likely catch bugs that we have not yet seen with the existing fuzz tests.

Describe the solution you'd like
Generate RegexAST structures using valid regular expression components.

Describe alternatives you've considered
None

Additional context
None