patch 8.2.1306: checking for first character of dict key is inconsistent

Problem:    Checking for first character of dict key is inconsistent.
Solution:   Add eval_isdictc(). (closes #6546)
diff --git a/src/testdir/test_vim9_expr.vim b/src/testdir/test_vim9_expr.vim
index d7072f3..5b46c32 100644
--- a/src/testdir/test_vim9_expr.vim
+++ b/src/testdir/test_vim9_expr.vim
@@ -1310,6 +1310,11 @@
 		  ])
   assert_equal(1, d
   	.one)
+  d = {'1': 1, '_': 2}
+  assert_equal(1, d
+  	.1)
+  assert_equal(2, d
+  	._)
 
   # getting the one member should clear the dict after getting the item
   assert_equal('one', #{one: 'one'}.one)
@@ -1330,10 +1335,16 @@
       vim9script
       let d = #{one:
       		'one',
-		two: 'two'}
+		two: 'two',
+		1: 1,
+		_: 2}
       assert_equal('one', d.one)
       assert_equal('one', d
                             .one)
+      assert_equal(1, d
+                            .1)
+      assert_equal(2, d
+                            ._)
       assert_equal('one', d[
 			    'one'
 			    ])