WYSIWG: Improved handling of cross-block code block creation

WYSIWG: Improved handling of cross-block code block creation

- Updated code content to get specific text selection instead of using
  node-based handling which could return the whole document when
  multiple top-level nodes were in selection.
- Simplified how code gets applied into the page to not be node based
  but use native editor methods to replace the selection. Allows
  creation from half-way through a block.

Tested on chrome+Firefox on Fedora 35.
Builds upon changes in #3246.
For #3200.