Private getters/setters

[RblSb]

var prop(private get, private set) syntax implementation.

There is many places where this read/write check can be done, so not sure if i did it correctly.

Currently errors highlights full obj.prop expression instead of prop part.

I also added unification from (private get/set) to (get/set) for interfaces, so you can have read-only field in interface and implement it as read-write, like in other langs.

Closes #3053

[Simn]

Nice!

I still can't remember if we ever agreed to actually do this, but I'll just make the executive decision to merge it if CI agrees. I scrolled through the changes and it looks good, though it's not super easy to review something like this because it affects so many places and at the same time doesn't since AccCall = AccPrivateCall most of the time.

One question: should public var foo(private get, private set):Int; really be allowed? I can't tell what the public is supposed to do here, the intent clearly seems to be to have this property not-public, so I wonder if this should be an error.

Also, a follow-up I'd like to look into later: I hate that FProp still works with plain strings... it's the parser's job to turn strings into something meaningful, so having these strings here and then matching them in the typer is stupid. We already deprecated anything other than get and set a long time ago, so let's clean this up.