Introduce the `AssignmentsCompletionProvider` to suggest variables

[Poitrin]

WHY are these changes introduced?

Fixes #659

WHAT is this pull request doing?

Introduce the AssignmentsCompletionProvider to suggest variables

How to test your changes?

1. Make sure that you are on the correct theme-check branch.
2. Test with Liquid code similar to this:

   {%- liquid
   assign product = all_products.first
   assign prod_uct = product
   %}
   {{ prod█ }}

3. When you start typing prod, do you see the following suggestions?
4. Does product only appear once? Do product and prod_uct display the correct description (namely the one about the globally available variable product)?

Add the following code below:

{%- liquid
  assign image1 = all_products.first.images.first
%}
{{ ima█ }}

1. When you type ima, does it suggest image1?
2. When you type ima in the first example (instead of prod), does it not suggest image1?