Allow block style in multiline scalars with trailing spaces

[Himura2la]

We use a DocFX-based documentation system, which stores markdown in YAML. DocFX-Flavoured Markdown has a syntax for adding a <br> tag, and it looks as follows:

line before br  <!-- notice the two spaces -->
line after br

this converts to

<p>
  line before br
  <br>
  line after br
</p>

When we store it in YAML and re-serialize the value, it's being converted from

summary: >-
  line before br  

  line after br

to

summary: "line before br  \nline after br"

, which is harder to read and edit.

I see the reason why trailing spaces are disallowed in blocks (they are hard to notice, and single-line notation makes them more explicit), although I suggest you guys to review our case and compare pros and cons.
Thanks!

P.S.: I'll be grateful for the hacktoberfest-accepted label on this PR.

[Himura2la]

OK, I see it breaks three tests from yaml-test-suite:

• https://github.com/yaml/yaml-test-suite/blob/master/test/H2RW.tml
• https://github.com/yaml/yaml-test-suite/blob/master/test/T26H.tml
• https://github.com/yaml/yaml-test-suite/blob/master/test/DWX9.tml

It's strange, because the https://github.com/eemeli/yaml library is also being tested for compatibility with this suite, and passes the tests yet allowing trailing whitespaces in blocks. I guess the current implementation needs more work... And tests... I'll continue the research once I have time

[Himura2la]

I hope it's ready, @aaubry, what do u think about it?

[aaubry]

Sorry for the delay. I think this is fine, since it doesn't break any test I don't see any problem with this. Thanks!