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
This uses the Emoji map from https://github.com/kyokomi/emoji -- but with a custom replacement implementation.
The built-in are fine for most use cases, but in Hugo we do care about pure speed.
The benchmarks below are skewed in Hugo's direction as the source and result is a byte slice,
Kyokomi's implementation works best with strings.
Curious: The easy-to-use `strings.Replacer` is also plenty fast.
```
BenchmarkEmojiKyokomiFprint-4 20000 86038 ns/op 33960 B/op 117 allocs/op
BenchmarkEmojiKyokomiSprint-4 20000 83252 ns/op 38232 B/op 122 allocs/op
BenchmarkEmojiStringsReplacer-4 100000 21092 ns/op 17248 B/op 25 allocs/op
BenchmarkHugoEmoji-4 500000 5728 ns/op 624 B/op 13 allocs/op
```
Fixes#1891
