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/shada.c

@@ -231,7 +231,7 @@ typedef struct {
   ShadaEntryType type;
   Timestamp timestamp;
   union {
-    Dictionary header;
+    Dict header;
     struct shada_filemark {
       char name;
       pos_T mark;
@@ -812,7 +812,7 @@ static inline void hms_dealloc(HistoryMergerState *const hms_p)
 
 /// Iterate over global variables
 ///
-/// @warning No modifications to global variable dictionary must be performed
+/// @warning No modifications to global variable Dict must be performed
 ///          while iteration is in progress.
 ///
 /// @param[in]   iter   Iterator. Pass NULL to start iteration.
@@ -2911,7 +2911,7 @@ static void shada_free_shada_entry(ShadaEntry *const entry)
     xfree(entry->data.unknown_item.contents);
     break;
   case kSDItemHeader:
-    api_free_dictionary(entry->data.header);
+    api_free_dict(entry->data.header);
     break;
   case kSDItemChange:
   case kSDItemJump:
@@ -3230,7 +3230,7 @@ shada_read_next_item_start:
   case kSDItemHeader:
     // TODO(bfredl): header is written to file and provides useful debugging
     // info. It is never read by nvim (earlier we parsed it back to a
-    // Dictionary, but that value was never used)
+    // Dict, but that value was never used)
     break;
   case kSDItemSearchPattern: {
     Dict(_shada_search_pat) *it = &entry->data.search_pattern;