duplicate 'systemConfig' added in TiddlerInFilePlugin

[PengjuYan]

I'm using the amazing TiddlerInFilePlugin as a helper to facilitate my plugin development.

There's a suspicious minor defect in it. When an external file is taken as a plugin, then the tag systemConfig will be appended no matter if one is already there. In some cases my tiddler will end up with duplicated systemConfig tags. I made the following modification in my repo, but did not thoroughly test it.

if (!tiddler.tags.includes('systemConfig')) {
    tiddler.tags.push('systemConfig');
}