Support coloring `lookup` keyword when used within a feature block

[miguelsousa]

Test case:

languagesystem DFLT dflt;

markClass [acute grave] <anchor 150 -10> @ALL_MARKS;

lookup CNTXT_PAIR_POS {
    pos T o -10;
    pos T c -12;
} CNTXT_PAIR_POS;
 
lookup CNTXT_MARK_TO_BASE {
    pos base o <anchor 250 450> mark @ALL_MARKS;
    pos base c <anchor 250 450> mark @ALL_MARKS;
} CNTXT_MARK_TO_BASE;

feature test {
    pos T' lookup CNTXT_PAIR_POS [o c]' @ALL_MARKS' lookup CNTXT_MARK_TO_BASE;
} test;