WIP: Attempt to fix the "box model" of code blocks

[NoahTheDuke]

An alternative to #1317. Follows the algorithm described in #1317 (comment):

1. If there is any text on the line containing the opening braces, ignore it for now.
2. Find the least indented line in the remaining lines, store the indent as the left edge of the box.
3. Remove the left edge indentation of each remaining line.
4. If there is any text on the line containing the opening braces, trim the whitespace and insert it at the front.

Adds tests of all examples from the above issue (including 9) which is considered invalid).