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/page_test.go

@@ -1497,6 +1497,45 @@ func TestChompBOM(t *testing.T) {
 	checkPageTitle(t, p, "Simple")
 }
 
+func TestPageWithEmoji(t *testing.T) {
+	for _, enableEmoji := range []bool{true, false} {
+		v := viper.New()
+		v.Set("enableEmoji", enableEmoji)
+		b := newTestSitesBuilder(t)
+		b.WithViper(v)
+
+		b.WithSimpleConfigFile()
+
+		b.WithContent("page-emoji.md", `---
+title: "Hugo Smile"
+---
+This is a :smile:.
+<!--more--> 
+
+Another :smile: This is :not: an emoji.
+
+`)
+
+		b.CreateSites().Build(BuildCfg{})
+
+		if enableEmoji {
+			b.AssertFileContent("public/page-emoji/index.html",
+				"This is a 😄",
+				"Another 😄",
+				"This is :not: an emoji",
+			)
+		} else {
+			b.AssertFileContent("public/page-emoji/index.html",
+				"This is a :smile:",
+				"Another :smile:",
+				"This is :not: an emoji",
+			)
+		}
+
+	}
+
+}
+
 // https://github.com/gohugoio/hugo/issues/5381
 func TestPageManualSummary(t *testing.T) {
 	b := newTestSitesBuilder(t)