Regex inconsistencies

Hello,
I've observed several inconsistencies between the regex pandoc uses when reading Typst documents and the regex Typst uses.

Here are a few of them:

1. Not all flags are supported. Typst regex supports the flags `i`, `m`, `s`, `u`, `x`. Of those, only `i` appears to be supported by Pandoc.
For example, `#(regex("(?m)a") in "A")` compiles in Typst, but doesn't in Pandoc (3.1.11.1 via try.pandoc.org), with the error `(line 1, column 2): parseRegex for Text.Regex.TDFA.Text failed:"({0,1}m)a" (line 1, column 4): unexpected '0' expecting an atom`.
- I especially miss the `m` (multiline) flag in order to be able to match the start of a line with `^` and the end of a line with `$`.
2. Unnamed capture groups are not supported: `#(regex("(?:x)") in "x")` compiles in Typst, but not in pandoc (`(line 1, column 2): parseRegex for Text.Regex.TDFA.Text failed:"({0,1}:x)" (line 1, column 4): unexpected '0' expecting an atom`).
- This is needed to avoid unnecessary capture groups in the output, and is frequently used across my packages.
3. Explicitly named capture groups are not supported: `#(regex("(?Px)") in "x")` compiles in Typst, but not in Pandoc (`(line 1, column 2): parseRegex for Text.Regex.TDFA.Text failed:"({0,1}Px)" (line 1, column 4): unexpected '0' expecting an atom`).

There are probably inconsistencies I haven't found yet as well, but they could be added to this issue as they are found.

Start by locating the Haskell regex handling used by the `regex` function and compare it with the listed Typst examples. Check flag support, capture-group syntax, and newline matching; done means the documented cases compile and produce matching results in Pandoc.

Written by the indexing model from the issue text.