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>

runtime/syntax/testdir/input/vim9_expr.vim

@@ -67,6 +67,33 @@ echo {
 # match as keys not scope dictionaries
 echo { b: 42, w: 42, t: 42, g: 42, l: 42, s: 42, a: 42, v: 42  }
 
+" Register
+
+echo @" @@
+echo @0 @1 @2 @3 @4 @5 @6 @7 @8 @9
+echo @a @b @c @d @e @f @g @h @i @j @k @l @m @n @o @p @q @r @s @t @u @v @w @x @y @z
+echo @A @B @C @D @E @F @G @H @I @J @K @L @M @N @O @P @Q @R @S @T @U @V @W @X @Y @Z
+echo @- @: @. @% @# @= @* @+ @~ @_ @/
+
+# read-only @:, @., @%, @~
+@" = "foo" 
+@0 = "foo"
+@1 = "foo"
+@9 = "foo"
+@a = "foo"
+@k = "foo"
+@z = "foo"
+@A = "foo"
+@K = "foo"
+@Z = "foo"
+@- = "foo"
+@# = "foo"
+@= = "foo"
+@* = "foo"
+@+ = "foo"
+@_ = "foo"
+@/ = "foo"
+
 # Operators
 
 # Ternary

runtime/syntax/testdir/input/vim_expr.vim

@@ -258,12 +258,30 @@ echo (42, 87) + foo
 
 " Register
 
-echo @" 
+echo @" @@
 echo @0 @1 @2 @3 @4 @5 @6 @7 @8 @9
-echo @-
 echo @a @b @c @d @e @f @g @h @i @j @k @l @m @n @o @p @q @r @s @t @u @v @w @x @y @z
 echo @A @B @C @D @E @F @G @H @I @J @K @L @M @N @O @P @Q @R @S @T @U @V @W @X @Y @Z
-echo @: @. @% @# @= @* @+ @~ @_ @/
+echo @- @: @. @% @# @= @* @+ @~ @_ @/
+
+" read-only @:, @., @%, @~
+let @" = "foo" 
+let @0 = "foo"
+let @1 = "foo"
+let @9 = "foo"
+let @a = "foo"
+let @k = "foo"
+let @z = "foo"
+let @A = "foo"
+let @K = "foo"
+let @Z = "foo"
+let @- = "foo"
+let @# = "foo"
+let @= = "foo"
+let @* = "foo"
+let @+ = "foo"
+let @_ = "foo"
+let @/ = "foo"
 
 " Operators