Add support for VapourSynth plugins through extras

[Ichunjo]

• Raises the minimum VapourSynth version to R73, so users can upgrade while keeping their existing setup.
  This will likely be increased to R75 in the next vsjetpack release.
• Adds several package extras. The basic ones will likely become required dependencies in the next version.
• Deprecate LSMAS, IMWRI, WWXD & cs.ImageSource

Current blocking factors

• vs-mlrt packages -> https://github.com/Jaded-Encoding-Thaumaturgy/vs-wheels

• Original dfttest: consider deprecating it? The GCC backend already supports all platforms.

• eedi2 -> consider deprecating?

• ffms2 -> Stalled:

  • Add meson build system FFMS/ffms2#471
  • Add meson cibuildwheel moi15moi/ffms2#8

[LightArrowsEXE]

• dfttest: I believe the original dfttest still had features that were missing in dfttest2?
• eedi3: In favour of dropping. I think most of the world has moved on to EEDI3 by now, and EEDI2 was only supported because "may as well". If there's no plans to make it easily available via PyPi like with the other packages, I don't see much reason for us to continue supporting it going forward.

[Ichunjo]

ffms2 is now available on the vs-wheels index while we're waiting for it to be available on pypi

[Ichunjo]

Marked eedi2 and dfttest as deprecated