Server-side syntax highlighting for all code (#12047)

* Server-side syntax hilighting for all code

This PR does a few things:

* Remove all traces of highlight.js
* Use chroma library to provide fast syntax hilighting directly on the server
* Provide syntax hilighting for diffs
* Re-style both unified and split diffs views
* Add custom syntax hilighting styling for both regular and arc-green

Fixes #7729
Fixes #10157
Fixes #11825
Fixes #7728
Fixes #3872
Fixes #3682

And perhaps gets closer to #9553

* fix line marker

* fix repo search

* Fix single line select

* properly load settings

* npm uninstall highlight.js

* review suggestion

* code review

* forgot to call function

* fix test

* Apply suggestions from code review

suggestions from @silverwind thanks

Co-authored-by: silverwind <me@silverwind.io>

* code review

* copy/paste error

* Use const for highlight size limit

* Update web_src/less/_repository.less

Co-authored-by: Lauris BH <lauris@nix.lv>

* update size limit to 1MB and other styling tweaks

* fix highlighting for certain diff sections

* fix test

* add worker back as suggested

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Lauris BH <lauris@nix.lv>

vendor/github.com/dlclark/regexp2/syntax/escape.go

@@ -0,0 +1,94 @@
+package syntax
+
+import (
+	"bytes"
+	"strconv"
+	"strings"
+	"unicode"
+)
+
+func Escape(input string) string {
+	b := &bytes.Buffer{}
+	for _, r := range input {
+		escape(b, r, false)
+	}
+	return b.String()
+}
+
+const meta = `\.+*?()|[]{}^$# `
+
+func escape(b *bytes.Buffer, r rune, force bool) {
+	if unicode.IsPrint(r) {
+		if strings.IndexRune(meta, r) >= 0 || force {
+			b.WriteRune('\\')
+		}
+		b.WriteRune(r)
+		return
+	}
+
+	switch r {
+	case '\a':
+		b.WriteString(`\a`)
+	case '\f':
+		b.WriteString(`\f`)
+	case '\n':
+		b.WriteString(`\n`)
+	case '\r':
+		b.WriteString(`\r`)
+	case '\t':
+		b.WriteString(`\t`)
+	case '\v':
+		b.WriteString(`\v`)
+	default:
+		if r < 0x100 {
+			b.WriteString(`\x`)
+			s := strconv.FormatInt(int64(r), 16)
+			if len(s) == 1 {
+				b.WriteRune('0')
+			}
+			b.WriteString(s)
+			break
+		}
+		b.WriteString(`\u`)
+		b.WriteString(strconv.FormatInt(int64(r), 16))
+	}
+}
+
+func Unescape(input string) (string, error) {
+	idx := strings.IndexRune(input, '\\')
+	// no slashes means no unescape needed
+	if idx == -1 {
+		return input, nil
+	}
+
+	buf := bytes.NewBufferString(input[:idx])
+	// get the runes for the rest of the string -- we're going full parser scan on this
+
+	p := parser{}
+	p.setPattern(input[idx+1:])
+	for {
+		if p.rightMost() {
+			return "", p.getErr(ErrIllegalEndEscape)
+		}
+		r, err := p.scanCharEscape()
+		if err != nil {
+			return "", err
+		}
+		buf.WriteRune(r)
+		// are we done?
+		if p.rightMost() {
+			return buf.String(), nil
+		}
+
+		r = p.moveRightGetChar()
+		for r != '\\' {
+			buf.WriteRune(r)
+			if p.rightMost() {
+				// we're done, no more slashes
+				return buf.String(), nil
+			}
+			// keep scanning until we get another slash
+			r = p.moveRightGetChar()
+		}
+	}
+}