vim-patch:9.1.0329: String interpolation fails for Dict type (#28335)

Problem:  String interpolation fails for Dict type
Solution: Support Dict data type properly, also support :put =Dict
          (without having to convert it to string() first)
          (Yegappan Lakshmanan)

fixes: vim/vim#14529
closes: vim/vim#14541

f01493c550

Co-authored-by: Yegappan Lakshmanan <yegappan@yahoo.com>

test/old/testdir/test_expr.vim

@@ -904,6 +904,10 @@ func Test_string_interp()
     endif
     call assert_equal(0, tmp)
 
+    #" Dict interpolation
+    VAR d = {'a': 10, 'b': [1, 2]}
+    call assert_equal("{'a': 10, 'b': [1, 2]}", $'{d}')
+
     #" Stray closing brace.
     call assert_fails('echo $"moo}"', 'E1278:')
     #" Undefined variable in expansion.