Add support for CoNLL-U framework

Language
CoNLL-U is an annotation framework for syntactically annotated treebank files in the Universal Dependencies project, which contains 200+ datasets for 100+ languages.

The framework is relaitvely simple, so syntax highlighting should be easy to configure for it.

Additional resources

Example CoNLL-U content:

# sent_id = 1
# text = They buy and sell books.
1   They     they    PRON    PRP    Case=Nom|Number=Plur               2   nsubj   2:nsubj|4:nsubj   _
2   buy      buy     VERB    VBP    Number=Plur|Person=3|Tense=Pres    0   root    0:root            _
3   and      and     CCONJ   CC     _                                  4   cc      4:cc              _
4   sell     sell    VERB    VBP    Number=Plur|Person=3|Tense=Pres    2   conj    0:root|2:conj     _
5   books    book    NOUN    NNS    Number=Plur                        2   obj     2:obj|4:obj       SpaceAfter=No
6   .        .       PUNCT   .      _                                  2   punct   2:punct           _

# sent_id = 2
# text = I have no clue.
1   I       I       PRON    PRP   Case=Nom|Number=Sing|Person=1     2   nsubj   _   _
2   have    have    VERB    VBP   Number=Sing|Person=1|Tense=Pres   0   root    _   _
3   no      no      DET     DT    PronType=Neg                      4   det     _   _
4   clue    clue    NOUN    NN    Number=Sing                       2   obj     _   SpaceAfter=No
5   .       .       PUNCT   .     _                                 2   punct   _   _

Official documentation:
UD home page

Existing highlighting implementations for other environments:
VSCode
SublimeText
Atom (RIP)

Start by reading the official Universal Dependencies format documentation and comparing the linked VSCode, SublimeText, and Atom highlighting implementations. Then inspect PrismJS's existing language definitions and testing approach. Done means CoNLL-U examples receive appropriate syntax highlighting without breaking existing grammars.