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_main_test.go

@@ -0,0 +1,40 @@
+// Copyright 2018 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package pageparser
+
+import (
+	"fmt"
+	"testing"
+)
+
+func TestMain(t *testing.T) {
+	t.Parallel()
+
+	var mainTests = []lexerTest{
+		{"emoji #1", "Some text with :emoji:", []Item{nti(tText, "Some text with "), nti(TypeEmoji, ":emoji:"), tstEOF}},
+		{"emoji #2", "Some text with :emoji: and some text.", []Item{nti(tText, "Some text with "), nti(TypeEmoji, ":emoji:"), nti(tText, " and some text."), tstEOF}},
+		{"looks like an emoji #1", "Some text and then :emoji", []Item{nti(tText, "Some text and then "), nti(tText, ":"), nti(tText, "emoji"), tstEOF}},
+		{"looks like an emoji #2", "Some text and then ::", []Item{nti(tText, "Some text and then "), nti(tText, ":"), nti(tText, ":"), tstEOF}},
+		{"looks like an emoji #3", ":Some :text", []Item{nti(tText, ":"), nti(tText, "Some "), nti(tText, ":"), nti(tText, "text"), tstEOF}},
+	}
+
+	for i, test := range mainTests {
+		items := collectWithConfig([]byte(test.input), false, lexMainSection, Config{EnableEmoji: true})
+		if !equal(items, test.items) {
+			got := crLfReplacer.Replace(fmt.Sprint(items))
+			expected := crLfReplacer.Replace(fmt.Sprint(test.items))
+			t.Errorf("[%d] %s: got\n\t%v\nexpected\n\t%v", i, test.name, got, expected)
+		}
+	}
+}