Expr: Implement string in table[pattern] of X

Not sure how useful this is (and the implementation isn't optimized in
any way), but seems reasonable for consistency.

Vern suggested that set[pattern] can already be achieved via
set_to_regex(), so left out any set[pattern] variants.

testing/btest/Baseline/language.pattern-tables-when/out

@@ -4,3 +4,5 @@ populate_a()
 gotcha a, [42]
 populate_b()
 gotcha b, [4242]
+populate_c()
+gotcha c, [4711]