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 a breaking change.

src/nvim/generators/gen_api_ui_events.lua

@@ -205,8 +205,9 @@ for _, ev in ipairs(events) do
     ev_exported[attr] = ev[attr]
   end
   for _, p in ipairs(ev_exported.parameters) do
-    if p[1] == 'HlAttrs' then
-      p[1] = 'Dict'
+    if p[1] == 'HlAttrs' or p[1] == 'Dict' then
+      -- TODO(justinmk): for back-compat, but do clients actually look at this?
+      p[1] = 'Dictionary'
     end
   end
   if not ev.noexport then