Support filling line comments in c-ts-mode

Support filling line comments in c-ts-mode

Now we handle line comments separately because for line comments, each
line is an individual node, unlike block comments, where the whole
block comment is a single node.  fill-comment-paragraph turns out to
do the job perfectly fine.

* lisp/progmodes/c-ts-mode.el:
(c-ts-mode--fill-paragraph): Extract out most parts into
c-ts-mode--fill-block-comment.
(c-ts-mode--fill-block-comment): New extracted function.