js: use prefixed names for functions and global symbols (#11387)

vlib/builtin/js/builtin.js.v

@@ -1,6 +1,8 @@
 module builtin
 
+import strings
 // used to generate JS throw statements.
+
 pub fn js_throw(s any) {
 	#throw s
 }
@@ -68,3 +70,12 @@ pub fn unwrap(opt string) string {
 
 	return res
 }
+
+pub fn (r rune) str() string {
+	res := ''
+	mut sb := strings.new_builder(5)
+	#res.str = r.valueOf().toString()
+	sb.write_string(res)
+
+	return sb.str()
+}