[win] Reopen and update Avisynth support

[inkydragon]

The DLL is packaged but the compilation option is not enabled：

Aegisub/tools/win-installer-setup.ps1

Line 41 in 0dffcec

$avsReleases = Invoke-WebRequest "https://api.github.com/repos/AviSynth/AviSynthPlus/releases/latest" -UseBasicParsing | ConvertFrom-Json

Aegisub/packages/win_installer/portable/create-portable.ps1

Lines 55 to 58 in 0dffcec

	Write-Output 'Copying - codecs\Avisynth'
	Copy-New-Item $InstallerDepsDir\AvisynthPlus64\x86-64\DevIL.dll $PortableOutputDir
	Copy-New-Item $InstallerDepsDir\AvisynthPlus64\x86-64\AviSynth.dll $PortableOutputDir
	Copy-New-Item $InstallerDepsDir\AvisynthPlus64\x86-64\plugins\DirectShowSource.dll $PortableOutputDir

Aegisub/meson_options.txt

Line 9 in 0dffcec

option('avisynth', type: 'feature', description: 'AviSynth video source')

We also need to update header files:

Aegisub/src/avisynth.h

Line 42 in 0dffcec

enum { AVISYNTH_INTERFACE_VERSION = 3 };

AVISYNTH_INTERFACE_VERSION = 8 —— AviSynthPlus/avisynth.h at master · AviSynth/AviSynthPlus

Maybe add the header files as a dependency.
Headers for AviSynthPlus

[CoffeeFlux]

I ran into build issues when trying to update them and decided to deprioritize working on the Avisynth provider since ffmpeg is good enough for most people. If you do get it working, it should include an option in the meson_options.txt file to turn it on/off (and maybe be off by default?)