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/testdir/input/vim_ex_import.vim

@@ -0,0 +1,119 @@
+" Vim :import command
+" VIM_TEST_SETUP hi link vimImportName Todo
+
+
+import "foo.vim"
+impor v:true ? "foo.vim" : "bar.vim"
+
+import v:true ?
+      \ "foo.vim" :
+      \ "bar.vim"
+
+import v:true
+      \ ? "foo.vim"
+      \ : "bar.vim"
+
+import v:true ?
+      "\ comment 
+      \ "foo.vim" :
+      "\ comment 
+      \ "bar.vim"
+
+import v:true
+      "\ comment 
+      \ ? "foo.vim"
+      "\ comment
+      \ : "bar.vim"
+
+import "foo.vim" as bar
+import v:true ? "foo.vim" : "bar.vim" as baz
+
+import v:true ?
+      \ "foo.vim" :
+      \ "bar.vim"
+      \ as baz
+
+import v:true
+      \ ? "foo.vim"
+      \ : "bar.vim"
+      \ as baz
+
+import v:true ?
+      "\ comment 
+      \ "foo.vim" :
+      "\ comment 
+      \ "bar.vim"
+      "\ comment
+      \ as baz
+
+import v:true
+      "\ comment 
+      \ ? "foo.vim"
+      "\ comment
+      \ : "bar.vim"
+      "\ comment
+      \ as baz
+
+echo "Foo" | import "foo.vim"
+
+
+" autoload
+
+import autoload "foo.vim"
+import autoload v:true ? "foo.vim" : "bar.vim"
+
+import autoload v:true ?
+      \ "foo.vim" :
+      \ "bar.vim"
+
+import autoload v:true
+      \ ? "foo.vim"
+      \ : "bar.vim"
+
+import autoload v:true ?
+      "\ comment 
+      \"foo.vim" :
+      "\ comment
+      \ "bar.vim"
+
+import autoload v:true
+      "\ comment 
+      \ ? "foo.vim"
+      "\ comment
+      \ : "bar.vim"
+
+import autoload "foo.vim" as bar
+import autoload v:true ? "foo.vim" : "bar.vim" as baz
+
+import autoload v:true ?
+      \ "foo.vim" :
+      \ "bar.vim"
+      \ as baz
+
+import autoload v:true
+      \ ? "foo.vim"
+      \ : "bar.vim"
+      \ as baz
+
+import autoload v:true ?
+      "\ comment 
+      \ "foo.vim" :
+      "\ comment 
+      \ "bar.vim"
+      "\ comment
+      \ as baz
+
+import autoload v:true
+      "\ comment 
+      \ ? "foo.vim"
+      "\ comment
+      \ : "bar.vim"
+      "\ comment
+      \ as baz
+
+
+" "as" keyword in expr
+
+let as = "modules/"
+import v:true ? as .. "foo.vim" : as .. "bar.vim" as other
+