Add more hooks for plugins

[ang-zeyu]

Is your request related to a problem?

Work done here for a rough idea: #968

Plugins modify the content currently only through 2 hooks, pre/postRender.
This is very expensive however, as it involves parsing each page an additional time for each plugin there is.

Describe the solution you'd like

Since element processing in componentParser/Preprocessor is now split cleanly on a per-element basis, and these elements are presented in a familiar format that is from htmlParser2, we could provide hooks in these places to let plugins tap directly into the processes(s) here.

Cons:

• It's still a less clean solution than strictly abiding by passing the string content, but
  • Reparsing content for every plugin there is isn't really scalable in achieving Strong support for diagrams #984
• componentPreprocessor has some amount of cyclic dependencies with Parser still that shouldn't need to be exposed to plugins ideally. We could find a solution to this first, or only do it for componentParser

Describe alternatives you've considered

na

Additional context

na