Perform a regex substitution in the substitute plugin

[nichobi]

Description

This utilises regex substitution in the substitute plugin. The previous approach only used regex to match the pattern, then replaced it with a static string. This change allows more complex substitutions, where the output depends on the input.

Example use case

Say we want to keep only the first artist of a multi-artist credit, as in the following list:

Neil Young & Crazy Horse -> Neil Young
Michael Hurley, The Holy Modal Rounders, Jeffrey Frederick & The Clamtones -> Michael Hurley
James Yorkston and the Athletes -> James Yorkston

This would previously have required three separate rules, one for each resulting artist. By using a regex substitution, we can get the desired behaviour in a single rule:

substitute:
  ^(.*?)(,| &| and).*: \1

(Capture the text until the first , & or and, then use that capture group as the output)

Notes

I've kept the previous behaviour of only applying the first matching rule, but I'm not 100% sure it's the ideal approach.
I can imagine both cases where you want to apply several rules in sequence and cases where you want to stop after the first match.

To Do

• Documentation. (If you've added a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
• Changelog. (Add an entry to docs/changelog.rst to the bottom of one of the lists near the top of the document.)
• Tests. (Very much encouraged but not strictly required.)

[github-actions]

Thank you for the PR! The changelog has not been updated, so here is a friendly reminder to check if you need to add an entry.

[Serene-Arc]

Hi! Thanks for the PR. Are you comfortable adding tests for this enhancement? Anything with regex should be heavily tested to make sure it behaves exactly as expected.