feat: add Show link connection stroke setting

[henrikvilhelmberglund]

Hi, my first contribution here so things might look odd, please correct me if I'm doing something silly.

This PR uses render_connections_border to allow the user to hide the black link connection strokes. This is useful when using a low alpha value for the link colors since this causes the links to turn really black and a bit ugly.

Show link connection stroke enabled (default value):

Show link connection stroke disabled:

~~This PR is dependent on https://github.com/Comfy-Org/litegraph.js/pull/1039~~ I was being silly, now uses already exposed variable.

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