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/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
+