fix(rust,python): handle edge-case with string-literal replacement when the replace value looks like a capture group

[alexander-beedie]

Closes #6755.

Addresses edge-case with literal string replacement when using the regex engine. If the replacement pattern looks like (or contains) a capture group string you could get some unexpected/failed substitutions...

Example

import polars as pl
s = pl.Series( "srs", ["."] )
s.str.replace_all( ".", "$0", literal=True )[0]

Before: '.' (applied capture group, despite "literal=True")
After: '$0' (capture group syntax ignored, literal properly replaced)

Also

• Added test coverage for replacement patterns of this type.
• Making explicit use of NoExpand for literal replacement should also be marginally faster in general, as no capture groups have to be searched for at all:
  • https://docs.rs/regex/latest/regex/struct.NoExpand.html