[Yaml] fixed bugs with folded scalar parsing (2.1 branch)

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

This is a re-do of #7445, which was submitted against 2.0, which is no longer supported. Additionally, the previous PR broke some tests because literal tabs were accidentally changed to spaces, and GitHub's diff engine seemingly ignored the change, although the differences showed up when using git diff directly. This PR leaves the literals untouched, and hence the tests should now pass.

**From the original PR:**

I found some more bugs with the parsing of folded scalars. I'd also made some mistakes in the tests introduced by me in #6785. This PR fixes the incorrect tests, and introduces more related tests.

I had to completely rewrite the `Parser::parseFoldedScalar()` method. I think it came out simpler in the end - less 'special cases'. I've done some basic profiling by running the parser tests on repeat and haven't noticed any difference in parsing performance.