runtime(vim): Update base-syntax, match unamed register alias

The unamed register may be referenced as both @" and @@.

Remove the unused vimPlainRegister syntax group.

fixes: #17603.
closes: #17605

Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/syntax/vim.vim b/runtime/syntax/vim.vim
index e4aa216..a9c030b 100644
--- a/runtime/syntax/vim.vim
+++ b/runtime/syntax/vim.vim
@@ -2,7 +2,7 @@
 " Language:	   Vim script
 " Maintainer:	   Hirohito Higashi <h.east.727 ATMARK gmail.com>
 "	   Doug Kearns <dougkearns@gmail.com>
-" Last Change:	   2025 Jun 24
+" Last Change:	   2025 Jun 25
 " Former Maintainer: Charles E. Campbell
 
 " DO NOT CHANGE DIRECTLY.
@@ -1016,8 +1016,7 @@
 
 syn match	vimRegister	'[^,;[{: \t]\zs"[a-zA-Z0-9.%#:_\-/]\ze[^a-zA-Z_":0-9]'
 syn match	vimRegister	'@"'
-syn match	vimPlainRegister contained	'"[a-zA-Z0-9\-:.%#*+=]'
-syn match	vimLetRegister	contained	'@["0-9\-a-zA-Z:.%#=*+~_/]'
+syn match	vimLetRegister	contained	'@["@0-9\-a-zA-Z:.%#=*+~_/]'
 
 syn match	vimAddress	",\zs[.$]"	skipwhite nextgroup=vimSubst1
 syn match	vimAddress	"%\ze\a"	skipwhite nextgroup=vimString,vimSubst1
@@ -2377,7 +2376,6 @@
  hi def link vimPattern	Type
  hi def link vimPerl	vimCommand
  hi def link vimPlainMark	vimMark
- hi def link vimPlainRegister	vimRegister
  hi def link vimProfile	vimCommand
  hi def link vimProfileArg	vimSpecial
  hi def link vimProfileBang	vimBang