[FEAT] Extract the Handlebars parser (#1713)

Extracts the parser to `@handlebars/parser`, where it can be shared
between different implementations. This means that e.g. Glimmer/Ember
will be able to iterate on new features without forcing Handlebars to
adopt them immediately, and vice versa. All implementors will be able to
absorb changes as it makes sense for them.

lib/handlebars.runtime.js

@@ -1,9 +1,9 @@
+import { Exception } from '@handlebars/parser';
 import * as base from './handlebars/base';
 
 // Each of these augment the Handlebars object. No need to setup here.
 // (This is done to easily share code between commonjs and browse envs)
 import SafeString from './handlebars/safe-string';
-import Exception from './handlebars/exception';
 import * as Utils from './handlebars/utils';
 import * as runtime from './handlebars/runtime';