runtime(vim): Update base-syntax, improve :import highlighting

- Match "autoload" as a keyword in :import commands.
- Match an expression argument for the filename.

closes: #15375

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

runtime/syntax/generator/gen_syntax_vim.vim

@@ -1,7 +1,7 @@
 " Vim syntax file generator
 " Language:		 Vim script
 " Maintainer:  Hirohito Higashi (h_east)
-" Last Change: 2025 May 16
+" Last Change: 2025 May 17
 
 let s:keepcpo= &cpo
 set cpo&vim
@@ -293,6 +293,7 @@ function s:get_vim_command_type(cmd_name)
 		grepadd
 		helpgrep
 		if
+		import
 		interface
 		insert
 		let