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/viml/parser/expressions.c

@@ -3005,8 +3005,7 @@ viml_pexpr_parse_end:
         break;
       case kExprNodeCurlyBracesIdentifier:
         // Until trailing "}" it is impossible to distinguish curly braces
-        // identifier and dictionary, so it must not appear in the stack like
-        // this.
+        // identifier and Dict, so it must not appear in the stack like this.
         abort();
       case kExprNodeInteger:
       case kExprNodeFloat:

src/nvim/viml/parser/expressions.h

@@ -216,7 +216,7 @@ typedef enum {
   /// kExprNodeCurlyBracesIdentifier.
   kExprNodeUnknownFigure,
   kExprNodeLambda,  ///< Lambda.
-  kExprNodeDictLiteral,  ///< Dictionary literal.
+  kExprNodeDictLiteral,  ///< Dict literal.
   kExprNodeCurlyBracesIdentifier,  ///< Part of the curly braces name.
   kExprNodeComma,  ///< Comma “operator”.
   kExprNodeColon,  ///< Colon “operator”.