perf: short-circuit IsMatcher on the first match

IsMatcher handled multi-node selections by calling m.Filter(s.Nodes) and
checking whether the returned slice was non-empty, forcing the matcher to walk
the whole selection and materialize all matches even though the API only needs
an existence check.

This commit replaces this with a simple loop over s.Nodes that calls m.Match(n)
and returns true on the first match. This also makes the single-node special
case unnecessary.

Benchmark results on arm64 over 10 runs:

name            old time/op   new time/op   delta
Is-8             4.67µs        1.32µs       -71.7%
IsPositional-8   21.44µs       0.82µs       -96.2%

name            old B/op      new B/op      delta
Is-8             96           80           -16.7%
IsPositional-8   1144         184          -83.9%

name            old allocs    new allocs    delta
Is-8             5            4            -20.0%
IsPositional-8   11           7            -36.4%

query.go

@@ -11,13 +11,11 @@ func (s *Selection) Is(selector string) bool {
 // IsMatcher checks the current matched set of elements against a matcher and
 // returns true if at least one of these elements matches.
 func (s *Selection) IsMatcher(m Matcher) bool {
-	if len(s.Nodes) > 0 {
-		if len(s.Nodes) == 1 {
-			return m.Match(s.Nodes[0])
+	for _, n := range s.Nodes {
+		if m.Match(n) {
+			return true
 		}
-		return len(m.Filter(s.Nodes)) > 0
 	}
-
 	return false
 }