Add template & compile functions for HTMLBars.

This provides a central location for any customizations we need to do to
the compiled templates from htmlbars-compiler.

Things like `isTop` and `isMethod` being set for example. These are
obviously, Ember concerns (neither thing really makes sense in
HTMLBars).

The node-land precompiler should now return the following template:

```javascript
export default Ember.HTMLBars.template(/* output from compileSpec */)
```
Merge pull request #9585 from rwjblue/add-template-and-compile

Add template & compile functions for HTMLBars.