js: fix wrong filename and invalid backend selected when using -o .js (#6777)

vlib/v/builder/js.v

@@ -45,7 +45,11 @@ pub fn (mut b Builder) compile_js() {
 		println('all .v files:')
 		println(files)
 	}
-	b.build_js(files, b.pref.out_name + '.js')
+	mut name := b.pref.out_name
+	if !name.ends_with('.js') {
+		name += '.js'
+	}
+	b.build_js(files, name)
 }
 
 fn (mut b Builder) run_js() {

vlib/v/pref/pref.v

@@ -316,6 +316,9 @@ pub fn parse_args(args []string) (&Preferences, string) {
 			}
 			'-o' {
 				res.out_name = cmdline.option(current_args, '-o', '')
+				if res.out_name.ends_with('.js') {
+					res.backend = .js
+				}
 				i++
 			}
 			'-b' {