Scaffolding for test262

- Adds a shallow git submodule for test262 as the tests aren't available   as an 
npm module - To run all tests: `yarn test262:all`. Note that this chunks up the  
 tests by test262 folder as there are over 50k+ tests and the test harness only 
accepts arrays of filepaths which exceeds arg limits - To run a specific test: 
`yarn test262 test262/test/folder/file.js`.   You can also pass globs which 
expand into an array of filepaths: `yarn test262 test262/test/folder/**/*.js` - 
More instructions for the test-harness can be found here: 
https://github.com/bterlson/test262-harness