Relocate procedural code in .module file to object-oriented filter

Problem/Motivation

In a few places in the code, there are remaining @todos for moving procedural code into an object oriented class that have been there for over a decade! Given that Drupal 13 will eliminate .module files and that most of the code that traditionally went in .modules files -- namely, hooks, need to be addressed for Drupal 12 compatibility, it makes sense for us to relocate the procedural code that are not hooks into a PHP Class.

This change will not affect compatibility with earlier versions of Drupal core that Pathologic's 2.0.x branch supports. Hook conversion should take place in a separate issue, since it will involve a somewhat different focus.

Proposed resolution

1. Convert _pathologic_filter(), _pathologic_replace(), _pathologic_replace_individual(), and to protected methods in the FilterPathologic.
2. Convert _pathologic_url_to_protocol_relative() to a public, static method, so that it can be used in direct calls in test execution.

Remaining tasks

N/A

User interface changes

N/A

API changes

N/A

Data model changes

N/A