cmd/v: implement help

cmd/tools/modules/vhelp/vhelp.v

@@ -0,0 +1,15 @@
+module vhelp
+
+import os
+
+pub fn show_topic(topic string) {
+	vexe := os.realpath(os.getenv('VEXE'))
+	vroot := os.dir(vexe)
+	target_topic := os.join_path(vroot,'cmd','v','internal','help','${topic}.txt')
+	content := os.read_file(target_topic) or {
+		eprintln('Unknown topic: $topic')
+		exit(1)
+	}
+	println(content)
+}
+