Add a "Set on fire" button to fire source

[laggron42]

This PR adds a button to the "Fire source" script (from InteractionSystem/Longbow) to set the component on fire without a nearby fire source.

I found this very useful when testing my game, and I think it could be a good addition to the repo!

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The usage of a button requires a second script, it can't be done from the same script. Please tell me if I should change the namespace or something else. Also worth noting I had to make the FireExposure method public.

[laggron42]

Here's a recording of the demo scene using the button

[zite]

Additions to the Interaction System are kind of out of scope at this point. But thanks for contributing!