grammar: reorder rules for better YAML rendering.

* lib/zile/grammar/yaml-bundle.lua (invalid_trailing_whitespace):
Match only trailing spaces and tabs; new-lines are fine!
(comment): Match comments first.
(unquoted_block): Allow multiple words in an entity tag.
Make flow style character optional, and add a capture scope.
(unquoted_string): Allow multiple words in an entity tag.
* lib/zz/default-color-bundle.lua: Adjust colors accordingly.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>