Add additional regexp functions

### Is your feature request related to a problem or challenge?

I would like to see the following regexp functions implemented. These exist in some, but not all, versions of PostgreSQL.

- [ ] [regexp_count()](https://pgpedia.info/r/regexp_count.html)
- [ ] [regexp_instr()](https://pgpedia.info/r/regexp_instr.html)
- [ ] [regexp_matches()](https://pgpedia.info/r/regexp_matches.html)
- [ ] [regexp_split_to_array()](https://pgpedia.info/r/regexp_split_to_array.html)
- [ ] [regexp_split_to_table()](https://pgpedia.info/r/regexp_split_to_table.html)
- [ ] [regexp_substr()](https://pgpedia.info/r/regexp_substr.html)

### Describe the solution you'd like

Implement these functions.

### Describe alternatives you've considered

These operations *can* be performed using the existing functions, so I am currently unblocked for my immediate use case but having these functions built in would be convenient.

### Additional context

We currently have the following regexp functions implemented. The source is in `datafusion/functions/src/regex/mod.rs`

[regexp_like()](https://pgpedia.info/r/regexp_like.html)
[regexp_match()](https://pgpedia.info/r/regexp_match.html)
[regexp_replace()](https://pgpedia.info/r/regexp_replace.html)

Start in datafusion/functions/src/regex/mod.rs and compare the existing regexp_like(), regexp_match(), and regexp_replace() implementations with the linked PostgreSQL references. Implement regexp_count(), regexp_instr(), regexp_matches(), regexp_split_to_array(), regexp_split_to_table(), and regexp_substr(); done means all six requested functions are available with the referenced PostgreSQL behavior.

Written by the indexing model from the issue text.