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.

src/nvim/api/keysets_defs.h

@@ -275,8 +275,8 @@ typedef struct {
   String reg;
   Boolean bang;
   Array args;
-  Dictionary magic;
-  Dictionary mods;
+  Dict magic;
+  Dict mods;
   Object nargs;
   Object addr;
   Object nextcmd;
@@ -293,7 +293,7 @@ typedef struct {
   Boolean silent;
   Boolean emsg_silent;
   Boolean unsilent;
-  Dictionary filter;
+  Dict filter;
   Boolean sandbox;
   Boolean noautocmd;
   Boolean browse;