[loader] Parse in more netcore hosting properties (#18850)
This includes some other minor cleanup to the netcore hosting functions.
Setting `assemblies_path` with `APP_PATHS` despite it also being set by `MONO_PATH` doesn't scare me too much because:
1) Anyone using `MONO_PATH` is probably not going through the hosting API and is using `mono-sgen` directly
2) The default host doesn't set `APP_PATHS` (and we've worked fine without it being set at all up until now
3) This is a temporary measure; we will have to refactor our preload hook when we want to properly support the other loader properties anyway
Co-authored-by: Ryan Lucia <ryan@luciaonline.net>