Parser: Change suffix to use ES6 default module export

- This export will be transpiled by Babel for the cjs distribution,
  but will enable others to  use a pure ES6 module distribution
- Instanbul: Ignore "parser.js" for coverage reporting. This file was ignored before
  via annotation, but this has no effect anymore due to the above change
- Remove istanbul annotation from `parser-prefix` (@nknapp)

Squashed by @nknapp

.istanbul.yml

@@ -1,2 +1,2 @@
 instrumentation:
-    excludes: ['**/spec/**']
+    excludes: ['**/spec/**', '**/handlebars/compiler/parser.js']

src/parser-prefix.js

@@ -1 +1 @@
-/* istanbul ignore next */
+// File ignored in coverage tests via setting in .istanbul.yml

src/parser-suffix.js

@@ -1,2 +1 @@
-exports.__esModule = true;
+export default handlebars;
-exports['default'] = handlebars;