Feature request: toggle-able file associations on installer

[OrangeChannel]

https://github.com/TypesettingTools/Aegisub/blob/75fc5f38d79ac2a776f6ccc452a8b7b3cd85c397/packages/win_installer/fragment_associations.iss#L38-L40

[1:43 AM] Ryan: there is significant benefit to being able to compile without lgpl so no, i would not say ffms2 should be assumed to be the only a/v provider [1:43 AM] Ryan: even if it's the default for most people [2:45 AM] Ryan: there are various providers you can build with [2:45 AM] Ryan: and it's not hard to add another if there was some reason you'd want to [2:46 AM] Ryan: it's probably worth going and making the extensions registered based on the providers available to people [2:46 AM] Ryan: the configuration flags should be imported into the installer [2:46 AM] Ryan: the avisynth one is iirc just calling directshowsource?