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