js: add initial support for runes (#12077)

vlib/builtin/js/rune.js.v

@@ -0,0 +1,31 @@
+module builtin
+
+import strings
+
+pub fn (ra []rune) string() string {
+	mut sb := strings.new_builder(ra.len)
+	sb.write_runes(ra)
+	res := sb.str()
+	return res
+}
+
+pub fn (c rune) repeat(count int) string {
+	if count < 0 {
+		panic('rune.repeat: count is negative: $count')
+	} else if count == 0 {
+		return ''
+	} else if count == 1 {
+		return c.str()
+	}
+	res := ''
+	#res.str = String.fromCharCode(c.val)
+
+	return res.repeat(count)
+}
+
+pub fn (c rune) str() string {
+	res := ''
+	#res.str = String.fromCharCode(c.val)
+
+	return res
+}