runtime(vim): Update base syntax, improve :@ highlighting

- Highlight :@ as a normal Ex command rather than something special.
- Fix erroneous matching of the Ex command as a register variable.

closes: #18624

Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/syntax/testdir/input/vim9_ex_commands.vim

@@ -3,8 +3,6 @@ vim9script
 
 
 # START NOT MATCHED
-:@
-:@@
 :Next
 :X
 # END NOT MATCHED
@@ -14,6 +12,8 @@ vim9script
 : help
  : help # FIXME
 
+:@
+
 :abbreviate
 :abclear
 :aboveleft

runtime/syntax/testdir/input/vim_ex_at.vim

@@ -0,0 +1,66 @@
+" Vim :@ command
+
+
+@0
+@1
+@2
+@3
+@4
+@5
+@6
+@7
+@8
+@9
+
+@a
+@k
+@z
+
+@"
+@.
+@=
+@*
+@+
+
+@:
+
+" repeats
+@
+@@
+
+@a | echo "..."
+@a " comment
+
+
+def Vim9Context()
+  :@0
+  :@1
+  :@2
+  :@3
+  :@4
+  :@5
+  :@6
+  :@7
+  :@8
+  :@9
+
+  :@a
+  :@k
+  :@z
+
+  :@"
+  :@.
+  :@=
+  :@*
+  :@+
+
+  :@:
+
+  # repeats
+  :@
+  :@@
+
+  :@a | echo "..."
+  :@a # comment
+enddef
+

runtime/syntax/testdir/input/vim_ex_commands.vim

@@ -2,8 +2,6 @@
 
 
 " START NOT MATCHED
-:@
-:@@
 :Next
 :Print
 :X
@@ -14,6 +12,8 @@
 : help
  : help # FIXME
 
+:@
+
 :append
     text
 .