Add support for SQL server regexp functions

[hadley]

Fixes #1671

@edward-burn could you please review this? It was mostly implemented by Claude code, which seems to be pretty good at writing SQL.

[edward-burn]

Have tested on SQL Server 2025 and all worked well!

The only thing I'm wondering is whether it could be clearer that this should all be tied to compatibility level as well as SQL Sever version?

At the moment if you have version 2025 but compatibility level is set to 160 (rather than 170)

Error in `dplyr::collect()`:
! Failed to collect lazy table.
Caused by error in `<fn>`:
! ODBC failed with error 00000 from [Microsoft][ODBC Driver 17
  for SQL Server][SQL Server].
✖ 'REGEXP_LIKE' is not a recognized built-in function name.

Whereas on SQL Server 2022 with compatibility level set to 160

Error in `str_starts()`:
! Only fixed patterns are supported on this backend.
Run `rlang::last_trace()` to see where the error occurred.

[hadley]

Yeah, I'm not too worried about that.