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/alecthomas/chroma/lexers/c/csharp.go

@@ -0,0 +1,51 @@
+package c
+
+import (
+	. "github.com/alecthomas/chroma" // nolint
+	"github.com/alecthomas/chroma/lexers/internal"
+)
+
+// CSharp lexer.
+var CSharp = internal.Register(MustNewLexer(
+	&Config{
+		Name:      "C#",
+		Aliases:   []string{"csharp", "c#"},
+		Filenames: []string{"*.cs"},
+		MimeTypes: []string{"text/x-csharp"},
+		DotAll:    true,
+		EnsureNL:  true,
+	},
+	Rules{
+		"root": {
+			{`^\s*\[.*?\]`, NameAttribute, nil},
+			{`[^\S\n]+`, Text, nil},
+			{`\\\n`, Text, nil},
+			{`//.*?\n`, CommentSingle, nil},
+			{`/[*].*?[*]/`, CommentMultiline, nil},
+			{`\n`, Text, nil},
+			{`[~!%^&*()+=|\[\]:;,.<>/?-]`, Punctuation, nil},
+			{`[{}]`, Punctuation, nil},
+			{`@"(""|[^"])*"`, LiteralString, nil},
+			{`\$@?"(""|[^"])*"`, LiteralString, nil},
+			{`"(\\\\|\\"|[^"\n])*["\n]`, LiteralString, nil},
+			{`'\\.'|'[^\\]'`, LiteralStringChar, nil},
+			{`[0-9](\.[0-9]*)?([eE][+-][0-9]+)?[flFLdD]?|0[xX][0-9a-fA-F]+[Ll]?`, LiteralNumber, nil},
+			{`#[ \t]*(if|endif|else|elif|define|undef|line|error|warning|region|endregion|pragma)\b.*?\n`, CommentPreproc, nil},
+			{`\b(extern)(\s+)(alias)\b`, ByGroups(Keyword, Text, Keyword), nil},
+			{`(abstract|as|async|await|base|break|by|case|catch|checked|const|continue|default|delegate|do|else|enum|event|explicit|extern|false|finally|fixed|for|foreach|goto|if|implicit|in|interface|internal|is|let|lock|new|null|on|operator|out|override|params|private|protected|public|readonly|ref|return|sealed|sizeof|stackalloc|static|switch|this|throw|true|try|typeof|unchecked|unsafe|virtual|void|while|get|set|new|partial|yield|add|remove|value|alias|ascending|descending|from|group|into|orderby|select|thenby|where|join|equals)\b`, Keyword, nil},
+			{`(global)(::)`, ByGroups(Keyword, Punctuation), nil},
+			{`(bool|byte|char|decimal|double|dynamic|float|int|long|object|sbyte|short|string|uint|ulong|ushort|var)\b\??`, KeywordType, nil},
+			{`(class|struct)(\s+)`, ByGroups(Keyword, Text), Push("class")},
+			{`(namespace|using)(\s+)`, ByGroups(Keyword, Text), Push("namespace")},
+			{`@?[_a-zA-Z]\w*`, Name, nil},
+		},
+		"class": {
+			{`@?[_a-zA-Z]\w*`, NameClass, Pop(1)},
+			Default(Pop(1)),
+		},
+		"namespace": {
+			{`(?=\()`, Text, Pop(1)},
+			{`(@?[_a-zA-Z]\w*|\.)+`, NameNamespace, Pop(1)},
+		},
+	},
+))