highlight `on (edgeql syntax)`

[1st1]

concept AbstractConstraintParent2:
    link name to str:
        delegated constraint unique on (lower(subject))

lower(subject) should be highlighted as EdgeQL

[1st1]

Same applies to constraints:

    link c_length_3 to constraint_length_2:
        constraint minlength(10, (SELECT ''), etc)

and

    link c_length_3 to constraint_length_2:
        constraint minlength(10, (SELECT ''), etc) on (SELECT 'abc')

and

constraint minlength(std::str='a') on (SELECT 'abc')

[vpetrovykh]

Done. The highlighter switches to EdgeQL for highlighting the contents of the on (...).