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

hugolib/shortcode.go

@@ -177,6 +177,16 @@ type shortcode struct {
 	pos       int // the position in bytes in the source file
 }
 
+func (s shortcode) innerString() string {
+	var sb strings.Builder
+
+	for _, inner := range s.inner {
+		sb.WriteString(inner.(string))
+	}
+
+	return sb.String()
+}
+
 func (sc shortcode) String() string {
 	// for testing (mostly), so any change here will break tests!
 	var params interface{}
@@ -363,7 +373,7 @@ func renderShortcode(
 	if sc.isInline {
 		templName := path.Join("_inline_shortcode", p.Path(), sc.name)
 		if sc.isClosing {
-			templStr := sc.inner[0].(string)
+			templStr := sc.innerString()
 
 			var err error
 			tmpl, err = p.s.TextTmpl.Parse(templName, templStr)