Move the emoji parsing to pageparser

This avoids double parsing the page content when `enableEmoji=true`.

This commit also adds some general improvements to the parser, making it in general much faster:

```bash
benchmark                     old ns/op     new ns/op     delta
BenchmarkShortcodeLexer-4     90258         101730        +12.71%
BenchmarkParse-4              148940        15037         -89.90%

benchmark                     old allocs     new allocs     delta
BenchmarkShortcodeLexer-4     456            700            +53.51%
BenchmarkParse-4              28             33             +17.86%

benchmark                     old bytes     new bytes     delta
BenchmarkShortcodeLexer-4     69875         81014         +15.94%
BenchmarkParse-4              8128          8304          +2.17%
```

Running some site benchmarks with Emoji support turned on:

```bash
benchmark                                                                                     old ns/op     new ns/op     delta
BenchmarkSiteBuilding/TOML,num_langs=3,num_pages=5000,tags_per_page=5,shortcodes,render-4     924556797     818115620     -11.51%

benchmark                                                                                     old allocs     new allocs     delta
BenchmarkSiteBuilding/TOML,num_langs=3,num_pages=5000,tags_per_page=5,shortcodes,render-4     4112613        4133787        +0.51%

benchmark                                                                                     old bytes     new bytes     delta
BenchmarkSiteBuilding/TOML,num_langs=3,num_pages=5000,tags_per_page=5,shortcodes,render-4     426982864     424363832     -0.61%
```

Fixes #5534

parser/pageparser/pageparser_intro_test.go

@@ -88,8 +88,8 @@ func TestFrontMatter(t *testing.T) {
 	}
 }
 
-func collect(input []byte, skipFrontMatter bool, stateStart stateFunc) (items []Item) {
-	l := newPageLexer(input, 0, stateStart)
+func collectWithConfig(input []byte, skipFrontMatter bool, stateStart stateFunc, cfg Config) (items []Item) {
+	l := newPageLexer(input, stateStart, cfg)
 	l.run()
 	t := l.newIterator()
 
@@ -103,6 +103,13 @@ func collect(input []byte, skipFrontMatter bool, stateStart stateFunc) (items []
 	return
 }
 
+func collect(input []byte, skipFrontMatter bool, stateStart stateFunc) (items []Item) {
+	var cfg Config
+
+	return collectWithConfig(input, skipFrontMatter, stateStart, cfg)
+
+}
+
 // no positional checking, for now ...
 func equal(i1, i2 []Item) bool {
 	if len(i1) != len(i2) {