Bug in `ttl` highlighting of abbreviations

[jeswr]

Originally raised in github-linguist/linguist#6352 (comment).

Describe the bug

The syntax highlighter supports the a abbreviation in Turtle; this works well in examples like

ex:jesse a ex:thing

However, it also matches against cases it shouldn't, for instance when the letter a is the suffix of a prefixed URI such as in the example below

ex:a ex:b ex:c

Expected behaviour

The a in ex:a should not be highlighted in the second example above.

Related discussion

Additional notes