Add support for [TOC] as alias for [[_TOC_]] in Markdown

• What does this MR do?
• Screenshots or Screencasts (strongly suggested)
• Does this MR meet the acceptance criteria?
  • Conformity
  • Availability and Testing

What does this MR do?

This MR adds support for the [TOC] tag as an alternative to [[_TOC_]] allowing more interoperability with other Markdown editors, as outlined in #14193 (closed).

Additionally, both tags (

• What does this MR do?
• Screenshots or Screencasts (strongly suggested)
• Does this MR meet the acceptance criteria?
  • Conformity
  • Availability and Testing

and [[_TOC_]]) are made case insensitive (so

• What does this MR do?
• Screenshots or Screencasts (strongly suggested)
• Does this MR meet the acceptance criteria?
  • Conformity
  • Availability and Testing

and [_toc_] will work as well).

Screenshots or Screencasts (strongly suggested)

becomes this:

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• [-] I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• [-] I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• [-] This change is backwards compatible across updates, or this does not apply.

Availability and Testing

As this is a backend change only, extensive cross-browser testing does not seem to be necessary.

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• [-] I have tested this MR in all supported browsers, or it's not needed.
• [-] I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Closes #14193 (closed)