Rewrite filtering logic using the DOM extension (instead of regular expressions)

[jan-san]

Trying to parse HTML with regular expressions is cumbersome, error-prone and ultimately futile.
A safer (and potentially more efficient) approach would be to use the DOM extension instead.

The DOM extension is not a hard requirement for running WordPress - based on a brief look into the code, all calls to DOM extension functionality in the WP core seem to be guarded with class_exists() checks to prevent errors. However, the extension is enabled in PHP by default (see here), and is listed as a required module for WordPress in the WP Extension Check command for WP-CLI.

The following would be necessary to move to the DOM extension:

• (optional) Implement a performance check for the filter_*() functions (to get a before/after comparison)
• Re-implement the filter_*() functions using the DOM extension
• Add a check to the plugin that disables all functionality and shows a warning if the DOM extension is not active