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/delegate.go

@@ -0,0 +1,137 @@
+package chroma
+
+import (
+	"bytes"
+)
+
+type delegatingLexer struct {
+	root     Lexer
+	language Lexer
+}
+
+// DelegatingLexer combines two lexers to handle the common case of a language embedded inside another, such as PHP
+// inside HTML or PHP inside plain text.
+//
+// It takes two lexer as arguments: a root lexer and a language lexer.  First everything is scanned using the language
+// lexer, which must return "Other" for unrecognised tokens. Then all "Other" tokens are lexed using the root lexer.
+// Finally, these two sets of tokens are merged.
+//
+// The lexers from the template lexer package use this base lexer.
+func DelegatingLexer(root Lexer, language Lexer) Lexer {
+	return &delegatingLexer{
+		root:     root,
+		language: language,
+	}
+}
+
+func (d *delegatingLexer) Config() *Config {
+	return d.language.Config()
+}
+
+// An insertion is the character range where language tokens should be inserted.
+type insertion struct {
+	start, end int
+	tokens     []Token
+}
+
+func (d *delegatingLexer) Tokenise(options *TokeniseOptions, text string) (Iterator, error) { // nolint: gocognit
+	tokens, err := Tokenise(Coalesce(d.language), options, text)
+	if err != nil {
+		return nil, err
+	}
+	// Compute insertions and gather "Other" tokens.
+	others := &bytes.Buffer{}
+	insertions := []*insertion{}
+	var insert *insertion
+	offset := 0
+	var last Token
+	for _, t := range tokens {
+		if t.Type == Other {
+			if last != EOF && insert != nil && last.Type != Other {
+				insert.end = offset
+			}
+			others.WriteString(t.Value)
+		} else {
+			if last == EOF || last.Type == Other {
+				insert = &insertion{start: offset}
+				insertions = append(insertions, insert)
+			}
+			insert.tokens = append(insert.tokens, t)
+		}
+		last = t
+		offset += len(t.Value)
+	}
+
+	if len(insertions) == 0 {
+		return d.root.Tokenise(options, text)
+	}
+
+	// Lex the other tokens.
+	rootTokens, err := Tokenise(Coalesce(d.root), options, others.String())
+	if err != nil {
+		return nil, err
+	}
+
+	// Interleave the two sets of tokens.
+	var out []Token
+	offset = 0 // Offset into text.
+	tokenIndex := 0
+	nextToken := func() Token {
+		if tokenIndex >= len(rootTokens) {
+			return EOF
+		}
+		t := rootTokens[tokenIndex]
+		tokenIndex++
+		return t
+	}
+	insertionIndex := 0
+	nextInsertion := func() *insertion {
+		if insertionIndex >= len(insertions) {
+			return nil
+		}
+		i := insertions[insertionIndex]
+		insertionIndex++
+		return i
+	}
+	t := nextToken()
+	i := nextInsertion()
+	for t != EOF || i != nil {
+		// fmt.Printf("%d->%d:%q   %d->%d:%q\n", offset, offset+len(t.Value), t.Value, i.start, i.end, Stringify(i.tokens...))
+		if t == EOF || (i != nil && i.start < offset+len(t.Value)) {
+			var l Token
+			l, t = splitToken(t, i.start-offset)
+			if l != EOF {
+				out = append(out, l)
+				offset += len(l.Value)
+			}
+			out = append(out, i.tokens...)
+			offset += i.end - i.start
+			if t == EOF {
+				t = nextToken()
+			}
+			i = nextInsertion()
+		} else {
+			out = append(out, t)
+			offset += len(t.Value)
+			t = nextToken()
+		}
+	}
+	return Literator(out...), nil
+}
+
+func splitToken(t Token, offset int) (l Token, r Token) {
+	if t == EOF {
+		return EOF, EOF
+	}
+	if offset == 0 {
+		return EOF, t
+	}
+	if offset == len(t.Value) {
+		return t, EOF
+	}
+	l = t.Clone()
+	r = t.Clone()
+	l.Value = l.Value[:offset]
+	r.Value = r.Value[offset:]
+	return
+}