Introduce new plugin versioning tag mod-version

New plugins should use the version tag:

-- mod-version: 1

The old version tag "-- lite-xl 1.16" will be considerer equivalent
to mod-version 1.

data/plugins/macro.lua

@@ -1,4 +1,4 @@
--- lite-xl 1.16
+-- mod-version:1 -- lite-xl 1.16
 local core = require "core"
 local command = require "core.command"
 local keymap = require "core.keymap"