Deprecate Property Fallback Lookup (no implicit this)

[johanrd]

See https://deprecations.emberjs.com/v3.x/#toc_this-property-fallback

Further work for another PR:
Read-only fields could perhaps be rewritten to use the @-syntax directly in the templates (for simplification).

E.g. resize-handle (and sort-indicator) could be a simple template-only component by using the @-syntax directly in the template:

-{{#if this.isResizable}}
+{{#if @columnsMeta.isResizable}}
  <div data-test-resize-handle class="et-header-resize-area">
  </div>
{{/if}}