Hello Emacs maintainers,

I noticed that the yaml-ts-mode fill-paragraph function doesn't play 
nicely with block nodes, making it difficult to tidy paragraphs. For 
example,

foo: |
   line-one
   line-two

Becomes

foo: | line-one line-two

Effectively undoing the block.

My proposed changes (see attached patch) fixes this behavior so that 
block nodes remain formatted correctly:

foo: |
   line-one line-two

It also plays nicely with longer pieces of text and multiple paragraphs.

I tried to base my changes off of the fill-paragraph function from 
c-ts-common, though the YAML version is much simpler since it doesn't 
need to account for comment insertion. I've not written a fill-paragraph 
function before so I'm sure I messed something up.

What do you think?