Switch to C++20 / Rebase on top of TypesettingTools/Aegisub

[arch1t3cht]

Tracking issue, see title. TypesettingTools/Aegisub switched to C++20 with various other refactors recently. Since ICU now requires C++17, this is getting more and more pressing. Some other dependencies like BestSource also require C++17.

It'll be tricky to rebase all the individual feature branches while ideally still keeping the old ones around for archiving purposes but I'll have to figure something out.