fix(markdown): don't delegate to HTML lexer

This causes issues whereby Markdown code fragments such as:

```
`<script>`
```

Are treated as HTML, resulting in errors and backtracking.

Note that this does mean that HTML in Markdown is not highlighted, but
we can potentially add rules to the Markdown lexer to solve that.

Fixes #1119

lexers/markdown.go

@@ -5,7 +5,7 @@ import (
 )
 
 // Markdown lexer.
-var Markdown = Register(DelegatingLexer(HTML, MustNewLexer(
+var Markdown = Register(MustNewLexer(
 	&Config{
 		Name:      "markdown",
 		Aliases:   []string{"md", "mkd"},
@@ -13,7 +13,7 @@ var Markdown = Register(DelegatingLexer(HTML, MustNewLexer(
 		MimeTypes: []string{"text/x-markdown"},
 	},
 	markdownRules,
-)))
+))
 
 func markdownRules() Rules {
 	return Rules{
@@ -40,7 +40,7 @@ func markdownRules() Rules {
 			{"`[^`]+`", LiteralStringBacktick, nil},
 			{`[@#][\w/:]+`, NameEntity, nil},
 			{`(!?\[)([^]]+)(\])(\()([^)]+)(\))`, ByGroups(Text, NameTag, Text, Text, NameAttribute, Text), nil},
-			{`.|\n`, Other, nil},
+			{`.|\n`, Text, nil},
 		},
 	}
 }

lexers/testdata/markdown.expected

@@ -1,18 +1,6 @@
 [
   {"type":"GenericHeading","value":"# about\n"},
-  {"type":"Text","value":"\n"},
-  {"type":"Punctuation","value":"\u003c"},
-  {"type":"NameTag","value":"div"},
-  {"type":"Text","value":" "},
-  {"type":"NameAttribute","value":"class"},
-  {"type":"Operator","value":"="},
-  {"type":"LiteralString","value":"\"html\""},
-  {"type":"Punctuation","value":"\u003e"},
-  {"type":"Text","value":"HTML"},
-  {"type":"Punctuation","value":"\u003c/"},
-  {"type":"NameTag","value":"div"},
-  {"type":"Punctuation","value":"\u003e"},
-  {"type":"Text","value":"\n\nMultiple "},
+  {"type":"Text","value":"\n\u003cdiv class=\"html\"\u003eHTML\u003c/div\u003e\n\nMultiple "},
   {"type":"GenericStrong","value":"**bold**"},
   {"type":"Text","value":" on the "},
   {"type":"GenericStrong","value":"**same line**"},