Don't output extra whitespace in YAML multiline (#993)

This resolves a particular issue with parsing YAML multiline, for
example:
```yaml
a: |
  multiline literal
  line 2
```

The regex used would capture the amount of indentation in the third
capture group and then use that as a kind of "status" to know which
lines are part of the indented multiline. However, because its a
captured group it has to be assigned a token which was `TextWhitespace`.
This meant that the indentation was outputted after the multiline,
technically it should be seen as an non-captured group, but then its no
longer to refer to it in the regex. Therefore I've gone with the
solution to add a new token, Ignore, which will not be emitted as a
token in the iterator, which can safely be used to make use of capture
groups but not have them show up in the output.

## Before

![image](https://github.com/user-attachments/assets/c29353c5-9e15-4f14-a733-57a60fb51910)

## After

![image](https://github.com/user-attachments/assets/57b5d129-a9d3-4b84-ae1f-dc05182b9ad3)

regexp.go

@@ -194,6 +194,9 @@ func (l *LexerState) Iterator() Token { // nolint: gocognit
 		for len(l.iteratorStack) > 0 {
 			n := len(l.iteratorStack) - 1
 			t := l.iteratorStack[n]()
+			if t.Type == Ignore {
+				continue
+			}
 			if t == EOF {
 				l.iteratorStack = l.iteratorStack[:n]
 				continue
@@ -243,6 +246,9 @@ func (l *LexerState) Iterator() Token { // nolint: gocognit
 	for len(l.iteratorStack) > 0 {
 		n := len(l.iteratorStack) - 1
 		t := l.iteratorStack[n]()
+		if t.Type == Ignore {
+			continue
+		}
 		if t == EOF {
 			l.iteratorStack = l.iteratorStack[:n]
 			continue