chore: add eslint-plugin-compat and eslint-plugin-es5

- tests are not compiled with babel and must thus be in
  es5
- we don't use polyfills, so we need to make sure no
  functions aren't used that are not supported by popular browsers. (like Object.assign in Safari and IE11)
- Boths are errors that usually only appear when running
  tests in SauceLabs, which happens only on _after_
  merging a PR.

spec/env/browser.js

@@ -9,7 +9,7 @@ var filename = 'dist/handlebars.js';
 if (global.minimizedTest) {
   filename = 'dist/handlebars.min.js';
 }
-var distHandlebars = fs.readFileSync(require.resolve(`../../${filename}`), 'utf-8');
+var distHandlebars = fs.readFileSync(require.resolve('../../' + filename), 'utf-8');
 vm.runInThisContext(distHandlebars, filename);
 
 global.CompilerContext = {