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/ast.js

@@ -126,7 +126,7 @@ describe('ast', function() {
   describe('whitespace control', function() {
     describe('parse', function() {
       it('mustache', function() {
-        let ast = Handlebars.parse('  {{~comment~}} ');
+        var ast = Handlebars.parse('  {{~comment~}} ');
 
         equals(ast.body[0].value, '');
         equals(ast.body[2].value, '');
@@ -142,7 +142,7 @@ describe('ast', function() {
 
     describe('parseWithoutProcessing', function() {
       it('mustache', function() {
-        let ast = Handlebars.parseWithoutProcessing('  {{~comment~}} ');
+        var ast = Handlebars.parseWithoutProcessing('  {{~comment~}} ');
 
         equals(ast.body[0].value, '  ');
         equals(ast.body[2].value, ' ');