Fix Fenced Code Syntax Highlighting in Markdown Leaking

[mjbvz]

Bug

This builds on #14488 to fix a number of issues where syntax highlighting leaks outside of code blocks in markdown files. A simple repo currently is this markdown:

**a**

```js
(
```

**a**

Which is incorrectly colored as:

Fix

Leverage the updated while rule logic from #14488 to break out of language syntax highlighting when the end of a code block is detected:

Also includes work to fix fenced code blocks where the size of the trailing fence is larger than the size of the starting fence:

```js
1;
`````

And cases such as:

```js
1;
                 `````

Possibly fixed issues include #9457, #8903, #8259, #13149, and others

This is not something we'd want to take into the current months release. I'd like to get some usage in insiders builds and target next month's release.

[mention-bot]

@mjbvz, thanks for your PR! By analyzing the history of the files in this pull request, we identified @kieferrm, @rebornix and @eklavyamirani to be potential reviewers.

[mjbvz]

We'll likely want to wait to take in microsoft/vscode-textmate#30 first before we merge this PR into VSCode itself. This change on its own without the vscode-textmate update breaks highlighting of fenced code blocks inside of block quotes:

> ```js
> 1;
> ```