patch 8.2.1359: Vim9: cannot assign to / register in Vim9 script

Problem:    Vim9: cannot assign to / register in Vim9 script.
Solution:   Adjust check for assignment in Vim9 script. (closes #6567)

Author: brammool

src/ex_docmd.c

@@ -3274,10 +3274,10 @@ find_ex_command(
     if (lookup != NULL)
     {
 	// Skip over first char for "&opt = val", "$ENV = val" and "@r = val".
-	char_u *pskip = (*eap->cmd == '&' || *eap->cmd == '$'
-				|| *eap->cmd == '@') ? eap->cmd + 1 : eap->cmd;
+	char_u *pskip = (*eap->cmd == '&' || *eap->cmd == '$')
+						     ? eap->cmd + 1 : eap->cmd;
 
-	if (vim_strchr((char_u *)"{('[\"", *p) != NULL
+	if (vim_strchr((char_u *)"{('[\"@", *p) != NULL
 	       || ((p = to_name_const_end(pskip)) > eap->cmd && *p != NUL))
 	{
 	    int oplen;
@@ -3336,6 +3336,8 @@ find_ex_command(
 	    // Recognize an assignment if we recognize the variable name:
 	    // "g:var = expr"
 	    // "var = expr"  where "var" is a local var name.
+	    if (*eap->cmd == '@')
+		p = eap->cmd + 2;
 	    oplen = assignment_len(skipwhite(p), &heredoc);
 	    if (oplen > 0)
 	    {

src/testdir/test_vim9_script.vim

@@ -423,6 +423,23 @@ def Test_assignment_vim9script()
     let ll =
           Func()
     assert_equal([1, 2], ll)
+
+    @/ = 'text'
+    assert_equal('text', @/)
+    @0 = 'zero'
+    assert_equal('zero', @0)
+    @1 = 'one'
+    assert_equal('one', @1)
+    @9 = 'nine'
+    assert_equal('nine', @9)
+    @- = 'minus'
+    assert_equal('minus', @-)
+    if has('clipboard_working')
+      @* = 'star'
+      assert_equal('star', @*)
+      @+ = 'plus'
+      assert_equal('plus', @+)
+    endif
   END
   CheckScriptSuccess(lines)
 enddef

src/version.c

@@ -754,6 +754,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1359,
 /**/
     1358,
 /**/