fix(completion): don't add backslashes to runtime pattern (#24296)

Problem:    Bashslashes added as regexp in runtime completion may be
            treated as path separator with some 'isfname' value.
Solution:   Make curly braces work for runtime completion and use it.

test/functional/lua/runtime_spec.lua

@@ -18,7 +18,10 @@ describe('runtime:', function()
     io.open(init, 'w'):close()  --  touch init file
     clear{args = {'-u', init}}
     exec('set rtp+=' .. plug_dir)
-    exec('set completeslash=slash')
+    exec([[
+      set completeslash=slash
+      set isfname+=(,)
+    ]])
   end)
 
   teardown(function()