Add change-header-lines action to dynamically change --header-lines

All input lines now enter the chunklist with sequential indices, and
header lines are excluded from matching via Pattern.startIndex and
PassMerger offset. This allows the number of header lines to be changed
at runtime with change-header-lines(N), transform-header-lines, and
bg-transform-header-lines actions.

- Remove EvtHeader event; header items are read directly from chunks
- Add startIndex to Pattern and PassMerger for skipping header items
- Add targetIndex field to Terminal for cursor repositioning across
  header-lines changes

Close #4659

src/matcher.go

@@ -174,7 +174,7 @@ func (m *Matcher) scan(request MatchRequest) MatchResult {
 		return MatchResult{m, m, false}
 	}
 	pattern := request.pattern
-	passMerger := PassMerger(&request.chunks, m.tac, request.revision)
+	passMerger := PassMerger(&request.chunks, m.tac, request.revision, pattern.startIndex)
 	if pattern.IsEmpty() {
 		return MatchResult{passMerger, passMerger, false}
 	}