Upgrade to .NET 8

[ScrubN]

Pros:

• More language features
• Native folder browser dialog
• Native Unix permissions handling (Mono.Posix can be removed)
• Source generated regex = noticeably faster startup times

Cons:

• For some stupid reason parts of the standard library that are used heavily by chat renders are significantly slower, leading to 30-100% slower render times.

[ScrubN]

I'm going to continue to investigate the root of the slowdowns. I don't know if I'll find anything, but hopefully I can.

[ScrubN]

In a barebones project, it seems that writing the bytes to stdin is not where the regression is.

[ScrubN]

The slowdown is caused by the new GC. Forcing clrgc.dll with environment variables solves the performance regressions.

[ScrubN]

Superseded by #1557