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/plugin/health_spec.lua

@@ -389,7 +389,7 @@ describe(':checkhealth window', function()
     command('file my_buff')
     command('checkhealth success1')
     -- define a function that collects all buffers in each tab
-    -- returns a dictionary like {tab1 = ["buf1", "buf2"], tab2 = ["buf3"]}
+    -- returns a dict like {tab1 = ["buf1", "buf2"], tab2 = ["buf3"]}
     source([[
         function CollectBuffersPerTab()
                 let buffs = {}