Improve control char rendering and escape button styling (#37094)

Follow-up to #37078.

- Use Unicode Control Pictures](U+2400-U+2421) to render C0 control characters
- Make it work in diff view too
- Replace escape warning emoji with SVG
- Align escape warning button with code lines

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/charset/charset.go

@@ -6,7 +6,10 @@ package charset
 import (
 	"bytes"
 	"io"
+	"regexp"
 	"strings"
+	"sync"
+	"unicode"
 	"unicode/utf8"
 
 	"code.gitea.io/gitea/modules/setting"
@@ -17,8 +20,19 @@ import (
 	"golang.org/x/text/transform"
 )
 
-// UTF8BOM is the utf-8 byte-order marker
-var UTF8BOM = []byte{'\xef', '\xbb', '\xbf'}
+var globalVars = sync.OnceValue(func() (ret struct {
+	utf8Bom []byte
+
+	defaultWordRegexp   *regexp.Regexp
+	ambiguousTableMap   map[string]*AmbiguousTable
+	invisibleRangeTable *unicode.RangeTable
+},
+) {
+	ret.utf8Bom = []byte{'\xef', '\xbb', '\xbf'}
+	ret.ambiguousTableMap = newAmbiguousTableMap()
+	ret.invisibleRangeTable = newInvisibleRangeTable()
+	return ret
+})
 
 type ConvertOpts struct {
 	KeepBOM           bool
@@ -105,7 +119,7 @@ func maybeRemoveBOM(content []byte, opts ConvertOpts) []byte {
 	if opts.KeepBOM {
 		return content
 	}
-	return bytes.TrimPrefix(content, UTF8BOM)
+	return bytes.TrimPrefix(content, globalVars().utf8Bom)
 }
 
 // DetectEncoding detect the encoding of content