v.gen.js: implement most of array and map methods in builtin/js (#10806)

vlib/builtin/js/map.js.v

@@ -0,0 +1,14 @@
+module builtin
+
+struct map {
+	m   JS.Map
+	len int
+}
+
+// Removes the mapping of a particular key from the map.
+[unsafe]
+pub fn (mut m map) delete(key voidptr) {
+	#m.m.delete(key)
+}
+
+pub fn (m &map) free() {}