Fix isAllowed of escapeStreamer (#22814) (#22837)

Backport #22814.

The use of `sort.Search` is wrong: The slice should be sorted, and
`return >= 0` doen't mean it exists, see the
[manual](https://pkg.go.dev/sort#Search).

Could be fixed like this if we really need it:

```diff
diff --git a/modules/charset/escape_stream.go b/modules/charset/escape_stream.go
index 823b63513..fcf1ffbc1 100644
--- a/modules/charset/escape_stream.go
+++ b/modules/charset/escape_stream.go
@@ -20,6 +20,9 @@ import (
 var defaultWordRegexp = regexp.MustCompile(`(-?\d*\.\d\w*)|([^\` + "`" + `\~\!\@\#\$\%\^\&\*\(\)\-\=\+\[\{\]\}\\\|\;\:\'\"\,\.\<\>\/\?\s\x00-\x1f]+)`)

 func NewEscapeStreamer(locale translation.Locale, next HTMLStreamer, allowed ...rune) HTMLStreamer {
+       sort.Slice(allowed, func(i, j int) bool {
+               return allowed[i] < allowed[j]
+       })
        return &escapeStreamer{
                escaped:                 &EscapeStatus{},
                PassthroughHTMLStreamer: *NewPassthroughStreamer(next),
@@ -284,14 +287,8 @@ func (e *escapeStreamer) runeTypes(runes ...rune) (types []runeType, confusables
 }

 func (e *escapeStreamer) isAllowed(r rune) bool {
-       if len(e.allowed) == 0 {
-               return false
-       }
-       if len(e.allowed) == 1 {
-               return e.allowed[0] == r
-       }
-
-       return sort.Search(len(e.allowed), func(i int) bool {
+       i := sort.Search(len(e.allowed), func(i int) bool {
                return e.allowed[i] >= r
-       }) >= 0
+       })
+       return i < len(e.allowed) && e.allowed[i] == r
 }
```

But I don't think so, a map is better to do it.

modules/charset/escape_stream.go

@@ -7,7 +7,6 @@ package charset
 import (
 	"fmt"
 	"regexp"
-	"sort"
 	"strings"
 	"unicode"
 	"unicode/utf8"
@@ -21,12 +20,16 @@ import (
 var defaultWordRegexp = regexp.MustCompile(`(-?\d*\.\d\w*)|([^\` + "`" + `\~\!\@\#\$\%\^\&\*\(\)\-\=\+\[\{\]\}\\\|\;\:\'\"\,\.\<\>\/\?\s\x00-\x1f]+)`)
 
 func NewEscapeStreamer(locale translation.Locale, next HTMLStreamer, allowed ...rune) HTMLStreamer {
+	allowedM := make(map[rune]bool, len(allowed))
+	for _, v := range allowed {
+		allowedM[v] = true
+	}
 	return &escapeStreamer{
 		escaped:                 &EscapeStatus{},
 		PassthroughHTMLStreamer: *NewPassthroughStreamer(next),
 		locale:                  locale,
 		ambiguousTables:         AmbiguousTablesForLocale(locale),
-		allowed:                 allowed,
+		allowed:                 allowedM,
 	}
 }
 
@@ -35,7 +38,7 @@ type escapeStreamer struct {
 	escaped         *EscapeStatus
 	locale          translation.Locale
 	ambiguousTables []*AmbiguousTable
-	allowed         []rune
+	allowed         map[rune]bool
 }
 
 func (e *escapeStreamer) EscapeStatus() *EscapeStatus {
@@ -257,7 +260,7 @@ func (e *escapeStreamer) runeTypes(runes ...rune) (types []runeType, confusables
 			runeCounts.numBrokenRunes++
 		case r == ' ' || r == '\t' || r == '\n':
 			runeCounts.numBasicRunes++
-		case e.isAllowed(r):
+		case e.allowed[r]:
 			if r > 0x7e || r < 0x20 {
 				types[i] = nonBasicASCIIRuneType
 				runeCounts.numNonConfusingNonBasicRunes++
@@ -283,16 +286,3 @@ func (e *escapeStreamer) runeTypes(runes ...rune) (types []runeType, confusables
 	}
 	return types, confusables, runeCounts
 }
-
-func (e *escapeStreamer) isAllowed(r rune) bool {
-	if len(e.allowed) == 0 {
-		return false
-	}
-	if len(e.allowed) == 1 {
-		return e.allowed[0] == r
-	}
-
-	return sort.Search(len(e.allowed), func(i int) bool {
-		return e.allowed[i] >= r
-	}) >= 0
-}