perf(emoji): optimize FindEmojiSubmatchIndex using slice-based Trie

[SudhanshuMatrix]

This pull request optimizes FindEmojiSubmatchIndex in Gitea's emoji package (modules/emoji/emoji.go) by replacing the strings.Replacer-based search with a slice-based trie and a constant-time starting-byte check (isStartingByte).

The new implementation avoids heap allocations during the search and reduces CPU overhead when rendering Markdown, particularly for plain text that does not contain emojis.

Verification

Verified with unit tests:

go test -count=1 ./modules/emoji/...

Benchmarks:

go test -bench=. -benchmem ./modules/emoji/...

Results

Benchmark	Before	After
BenchmarkFindEmojiSubmatchIndex	168.3 ns/op, 2 allocs/op	85.78 ns/op, 1 alloc/op
BenchmarkFindEmojiSubmatchIndexNoMatch	239.8 ns/op, 1 alloc/op	105.1 ns/op, 0 allocs/op

Benchmark Output

goos: linux
goarch: amd64
pkg: gitea.dev/modules/emoji
cpu: Intel(R) Core(TM) i5-7400 CPU @ 3.00GHz

BenchmarkFindEmojiSubmatchIndex-4              13498539        85.78 ns/op      16 B/op   1 allocs/op
BenchmarkFindEmojiSubmatchIndexNoMatch-4       11220450       105.1 ns/op        0 B/op   0 allocs/op
BenchmarkFindEmojiSubmatchIndexOld-4            6569360       168.3 ns/op      48 B/op   2 allocs/op
BenchmarkFindEmojiSubmatchIndexOldNoMatch-4     5026116       239.8 ns/op      32 B/op   1 allocs/op

[silverwind]

did some cleanups in 00e37e1.

[wxiaoguang]

did some cleanups in 00e37e1.

removed the useless and unclear test "a \U00000030\U0000fe0f\U000020e3 b".
\U00000030 is ASCII digit 0. Such case is already covered by many other cases.

---

OK, added it back and added more comments to explain the "keycap emoji" problem