Add regexp support for view filters

[piyooshm]

PR Description

Allow filtering with a Go regexp when gui.filterMode is regexp, or in substring/fuzzy mode by prefixing the filter with gui.regexpFilterPrefix (default re:). Invalid patterns match nothing; case rules match substring filters ((?i) when the pattern has no uppercase letter).

Add gui.regexpFilterPrefix so the prefix is configurable. Document in Config.md and Searching.md, update the JSON schema, and add unit and integration tests.

Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go generate ./...)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• If a new UserConfig entry was added, make sure it can be hot-reloaded (see here)
• Docs have been updated if necessary
• You've read through your own file changes for silly mistakes etc

Closes #5429