[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.

spec/env/runtime.js

@@ -22,7 +22,7 @@ vm.runInThisContext(
   filename
 );
 
-var parse = require('../../dist/cjs/handlebars/compiler/base').parse;
+var parse = require('@handlebars/parser').parse;
 var compiler = require('../../dist/cjs/handlebars/compiler/compiler');
 var JavaScriptCompiler = require('../../dist/cjs/handlebars/compiler/javascript-compiler');