refactor(api)!: rename Dictionary => Dict

In the api_info() output:

    :new|put =map(filter(api_info().functions, '!has_key(v:val,''deprecated_since'')'), 'v:val')
    ...

    {'return_type': 'ArrayOf(Integer, 2)', 'name': 'nvim_win_get_position', 'method': v:true, 'parameters': [['Window', 'window']], 'since': 1}

The `ArrayOf(Integer, 2)` return type didn't break clients when we added
it, which is evidence that clients don't use the `return_type` field,
thus renaming Dictionary => Dict in api_info() is not (in practice)
a breaking change.

test/functional/vimscript/json_functions_spec.lua

@@ -911,7 +911,7 @@ describe('json_encode() function', function()
     eq('[]', eval('json_encode(v:_null_list)'))
   end)
 
-  it('can dump NULL dictionary', function()
+  it('can dump NULL dict', function()
     eq('{}', eval('json_encode(v:_null_dict)'))
   end)