std: Add common to std/path

[kitsonk]

This PR add a utility that I have had to use before in the bundler to std/path but wanted to use it elsewhere, where the common path can be determined for a given set of paths.

For example:

import { common } from "https://deno.land/std/path/mod.ts";

const p = common(
  [
    "./deno/cli/js/deno.ts",
    "./deno/std/path/mod.ts",
    "./deno/cli/js/main.ts"
  ],
  "/"
);
console.log(p); // ./deno/

[ry]

LGTM