rules: add find and search rules

[Exirel]

Fixes #1757

Draft PR for now, until #1873 is at least approved.

Description

Based on #1873 this PR adds two new rules:

• Find rules: an anonymous rule that executes its handler for every instance found in the line, allowing to execute a function multiple times for a single trigger object
• Search rules: an anonymous rule that works like the original rule, but matches from anywhere in the line, allowing to execute a function exactly once on any word or pattern contained within a line, not just from its beginning

Checklist

• I have read CONTRIBUTING.md
• I can and do license this contribution under the EFLv2
• No issues are reported by make qa (runs make quality and make test)
• I have tested the functionality of the things this change touches

[dgw]

Rebased on your behalf, @Exirel. Look over it and mark ready for review if you're satisfied!

[Exirel]

Hmmmm not a huge fan of the rebase yourself, because it tends to mess with my local working copy when I pull. :(

[Exirel]

Re-signed & ready for review!

[dgw]

It's just little things. The tests prove that this (should) work as intended, so I focused on making sure the documentation was clear for plugin authors.

[dgw]

So, do we get to squash the docstring fixes in with the original changes, or is that a pain? x)

[Exirel]

Pain. It is.

[dgw]

In that case: Merge. It is.