Improve handling of the fenced code blocks

[oniboni]

Description

The plugin does not cover the current spec of Fenced code blocks¹

Background

This issue was initially raised and solved by @nolange ²³

Supports whitespace after the fence. ex:

stuff

Support more than 3 ticks or tildes for code blocks, this is still only a part of the variations in the wilds [1]

Finds the correct matching ending fence by using identation, this required adding some more groups to the regexp and the option MULTILINE. ex.

```` plantuml
    stuff containing ```
````

[1] - https://pandoc.org/MANUAL.html#fenced-code-blocks

[..]

Definition of Done

• merge Improve handling of the fenced code blocks #36 into new working branch
• add tests for usecases from ¹

Footnotes

1. https://spec.commonmark.org/0.31.2/#fenced-code-blocks ↩ ↩²

2. support whitespace after fenced block #34 ↩

3. Improve handling of the fenced code blocks #36 ↩