fix(11.1-07): check go fences, README src= and shell fences from the AST

- goLang follows the highlighter's chroma lookup, so golang and main.go need src=
- a src= fence in a module README is refused and never captioned
- shell fences inside callouts and lists are command-checked
- a fence with four or more leading spaces is an indented code block

internal/docsite/highlight.go

@@ -84,6 +84,15 @@ func highlight(lang, code string) string {
 	return b.String()
 }
 
+// goLang reports whether lang is the Go lexer the highlighter uses.
+// chroma's registry resolves names, aliases, case and file names, so
+// go, Go, GO, golang, Golang and main.go are Go. go-html-template,
+// go-text-template, text and an empty language are not.
+func goLang(lang string) bool {
+	lexer := lexerFor(lang)
+	return lexer != nil && lexer.Config().Name == "Go"
+}
+
 // lexerFor returns the chroma lexer for a fence language, or nil for
 // plain text and unknown languages.
 func lexerFor(lang string) chroma.Lexer {