A bug in highlighting YAML files when using multi-line strings

[aeschli]

1. Try using a multi-line string starting with a vertical bar.
2. Write some text on the next line.
3. Add text in quotes.
4. The last quote should be on a new line.
5. The rest of the text gets highlighted like a string.

EXAMPLES:

1. This produces the bug (see the attached image):

field1: 12
field2: |
  bash -c "
  git init
  "
field3:
  - 1234

2. This too:

field1: 12
field2: |
  bash -c "git init
  "
field3:
  - 1234

3. But this does not:

field1: 12
field2: |
  bash -c "git init
  && git add ."
field3:
  - 1234

So, the whole thing looks like it gets messed when a multi-line statement initialized with a vertical bar has a line with only quote.

Copied from original issue: microsoft/vscode#43065

[infininight]

Duplicate of #17. Slightly different trigger but the same cause, working to figure out a solution now.