feat(paste): support filter functions in tag-based paste config

[costajohnt]

Extend the paste processing logic to check filter functions when matching pasted elements to tools.

Previously, filter functions in pasteConfig.tags were only used during sanitization (via HTMLJanitor) but ignored during paste processing, causing all elements with a matching tag name to be treated as substitutable regardless of the filter.

Changes

• Add a filter field to TagSubstitute interface
• Store filter in getTagsConfig when the sanitization config is a function
• Add isTagSubstitutable helper that checks both tag name and filter
• Update processHTML, processElementNode, and containsAnotherToolTags to use the helper

Example

A tool can now use a filter function in pasteConfig.tags to match only specific elements:

static get pasteConfig() {
  return {
    tags: {
      SPAN: (el) => parseInt(el.style.fontWeight, 10) > 400
    }
  }
}

Previously, this filter was only checked during sanitization. Now it's also checked during paste processing, so only <span> elements that pass the filter will be assigned to the tool.

Closes #2959

[costajohnt]

Bumping this in case it got lost in the queue. The change is opt-in (filter functions are only used when configured), so existing paste configs are unaffected. Happy to address any feedback.

[neSpecc]

Overall seems ok. Please, add a corresponded test case. And write a line in a changelog

[costajohnt]

Totally understand if priorities have shifted here. Let me know if this is still something you'd consider, otherwise I'm happy to close it out.

[costajohnt]

Hi @neSpecc, following up on your review. Both asks are in, I just never said so here, sorry about that.

• Test case: 34864d3 adds a Cypress test in test/cypress/tests/copy-paste.cy.ts covering both branches of the filter function, one where the filter accepts the pasted node and one where it rejects it.
• Changelog: same commit adds an Improvement line under 2.31.6 in docs/CHANGELOG.md.

47e2e0b after it is just cleanup, dropping an unused tags var and stabilizing the new test.

I've also rebased onto current next, so this is running against the Node version fix from #3013. Anything else you'd like changed here?