parser/pageparser: Fix handling of commented out front matter

When the page parser was rewritten in 0.51, this was interpreted literally, but commented out front matter is used in the wild to "hide it from GitHub", e.g:

```
<!--
+++
title = "hello"
+++
-->
```

Fixes #5478

parser/pageparser/pagelexer.go

@@ -53,6 +53,8 @@ type pageLexer struct {
 	summaryDivider []byte
 	// Set when we have parsed any summary divider
 	summaryDividerChecked bool
+	// Whether we're in a HTML comment.
+	isInHTMLComment bool
 
 	lexerShortcodeState
 
@@ -120,7 +122,7 @@ var (
 	delimYAML         = []byte("---")
 	delimOrg          = []byte("#+")
 	htmlCommentStart  = []byte("<!--")
-	htmlCOmmentEnd    = []byte("-->")
+	htmlCommentEnd    = []byte("-->")
 )
 
 func (l *pageLexer) next() rune {
@@ -195,6 +197,15 @@ func (l *pageLexer) consumeCRLF() bool {
 	return consumed
 }
 
+func (l *pageLexer) consumeToNextLine() {
+	for {
+		r := l.next()
+		if r == eof || isEndOfLine(r) {
+			return
+		}
+	}
+}
+
 func (l *pageLexer) consumeSpace() {
 	for {
 		r := l.next()
@@ -206,6 +217,10 @@ func (l *pageLexer) consumeSpace() {
 }
 
 func lexMainSection(l *pageLexer) stateFunc {
+	if l.isInHTMLComment {
+		return lexEndFromtMatterHTMLComment
+	}
+
 	// Fast forward as far as possible.
 	var l1, l2 int
 
@@ -312,16 +327,15 @@ LOOP:
 		case r == byteOrderMark:
 			l.emit(TypeIgnore)
 		case !isSpace(r) && !isEndOfLine(r):
-			// No front matter.
 			if r == '<' {
 				l.backup()
 				if l.hasPrefix(htmlCommentStart) {
-					right := l.index(htmlCOmmentEnd)
-					if right == -1 {
-						return l.errorf("starting HTML comment with no end")
-					}
-					l.pos += right + len(htmlCOmmentEnd)
-					l.emit(TypeHTMLComment)
+					// This may be commented out front mattter, which should
+					// still be read.
+					l.consumeToNextLine()
+					l.isInHTMLComment = true
+					l.emit(TypeIgnore)
+					continue LOOP
 				} else {
 					if l.pos > l.start {
 						l.emit(tText)
@@ -341,6 +355,19 @@ LOOP:
 	return lexMainSection
 }
 
+func lexEndFromtMatterHTMLComment(l *pageLexer) stateFunc {
+	l.isInHTMLComment = false
+	right := l.index(htmlCommentEnd)
+	if right == -1 {
+		return l.errorf("starting HTML comment with no end")
+	}
+	l.pos += right + len(htmlCommentEnd)
+	l.emit(TypeIgnore)
+
+	// Now move on to the shortcodes.
+	return lexMainSection
+}
+
 func lexDone(l *pageLexer) stateFunc {
 
 	// Done!