patch 8.2.1429: Vim9: no error for missing white after : in dict

Problem:    Vim9: no error for missing white after : in dict.
Solution:   Check for white space. (closes #6671)  Also check that there is no
            white before the :.
diff --git a/src/testdir/test_vim9_expr.vim b/src/testdir/test_vim9_expr.vim
index 3d16053..a942dd4 100644
--- a/src/testdir/test_vim9_expr.vim
+++ b/src/testdir/test_vim9_expr.vim
@@ -1494,6 +1494,10 @@
   mixed = #{a: 234}
   mixed = #{}
 
+  call CheckDefFailure(["let x = #{a:8}"], 'E1069:')
+  call CheckDefFailure(["let x = #{a : 8}"], 'E1068:')
+  call CheckDefFailure(["let x = #{a :8}"], 'E1068:')
+
   call CheckDefFailure(["let x = #{8: 8}"], 'E1014:')
   call CheckDefFailure(["let x = #{xxx}"], 'E720:')
   call CheckDefFailure(["let x = #{xxx: 1", "let y = 2"], 'E722:')