v help: support `v help topics` to list all help .txt file names

cmd/v/help/default.txt

@@ -54,6 +54,7 @@ V supports the following commands:
 
 Use "v help <command>" for more information about a command, example: `v help build`, `v help build-c`, `v help build-native`
 Use "v help other" to see less frequently used commands.
+Use "v help topics" to see a list of all known help topics.
 
 Note: Help is required to write more help topics.
 Only build, new, init, doc, fmt, vet, run, test, watch, search, install, remove, update, bin2v, check-md are properly documented currently.

cmd/v/help/help.v

@@ -11,6 +11,8 @@ const (
 pub fn print_and_exit(topic string) {
 	vexe := pref.vexe_path()
 	vroot := os.dir(vexe)
+	topicdir := os.join_path(vroot, 'cmd', 'v', 'help')
+
 	for b in topic {
 		if (b >= `a` && b <= `z`) || b == `-` || (b >= `0` && b <= `9`) {
 			continue
@@ -20,11 +22,26 @@ pub fn print_and_exit(topic string) {
 	}
 	// `init` has the same help topic as `new`
 	name := if topic == 'init' { 'new' } else { topic }
-	target_topic := os.join_path(vroot, 'cmd', 'v', 'help', '${name}.txt')
+	if topic == 'topics' {
+		println(known_topics(topicdir))
+		exit(0)
+	}
+	target_topic := os.join_path(topicdir, '${name}.txt')
 	content := os.read_file(target_topic) or {
 		eprintln(help.unknown_topic)
+		eprintln(known_topics(topicdir))
 		exit(1)
 	}
 	println(content)
 	exit(0)
 }
+
+fn known_topics(topicdir string) string {
+	mut res := []string{}
+	res << 'Known help topics:'
+	topic_files := os.glob(os.join_path(topicdir, '*.txt')) or { [] }
+	mut topics := topic_files.map(os.file_name(it).replace('.txt', ''))
+	topics.sort()
+	res << topics.join(', ')
+	return res.join('')
+}