jsgen: implement js backend

cmd/tools/vfmt.v

@@ -18,6 +18,7 @@ import (
 struct FormatOptions {
 	is_l       bool
 	is_c       bool
+	is_js	   bool
 	is_w       bool
 	is_diff    bool
 	is_verbose bool
@@ -51,6 +52,7 @@ fn main() {
 	args := util.join_env_vflags_and_os_args()
 	foptions := FormatOptions{
 		is_c: '-c' in args
+		is_js: '-js' in args
 		is_l: '-l' in args
 		is_w: '-w' in args
 		is_diff: '-diff' in args
@@ -200,7 +202,7 @@ fn (foptions &FormatOptions) post_process_file(file string, formatted_file_path
 		return
 	}
 	is_formatted_different := fc != formatted_fc
-	if foptions.is_c {
+	if foptions.is_c || foptions.is_js {
 		if is_formatted_different {
 			eprintln('File is not formatted: $file')
 			exit(2)

cmd/v/v.v

@@ -160,6 +160,13 @@ fn parse_args(args []string) (&pref.Preferences, string) {
 				res.out_name  = cmdline.option(args, '-o', '')
 				i++
 			}
+			'-b' {
+				b := pref.backend_from_string(cmdline.option(args, '-b', 'c')) or {
+					continue
+				}
+				res.backend = b
+				i++
+			}
 			else {
 				mut should_continue := false
 				for flag_with_param in list_of_flags_with_param {