Fix newlines in tabbed codeblocks

[rafegoldberg]

RegEx Demo

Changes

The changes we pushed yesterday broke recognition of code tabs with multiple sequential blank lines:

```
this
tab
worked
```
```
but this

one broke
```

Yeek! I'm updating the RegEx pattern and adding some new tests for this functionality.

• 🐛 update tabbed code blocks RegEx pattern
• 🧬 test case for internal newlines
• 🧬 test subsequent, non-adjacent single code blocks
• 🧬 update single code block test expectations

Pattern Breakdown

Matching Groups	Look Ahead
Match anything between opening and closing ``` tags from the start of the input.	Extend match to the closing ``` tag followed by two new lines, or by a single new line that's not followed by an opening code tag itself, or to the end of the input string.

Jesus that's confusing to write out; fuckin’ RegEx, man...

[erunion]

👀