[ffigen] Fix category filtering bug

[liamappelbe]

ObjC categories map pretty neatly to Dart extension methods. The only issue we run into is methods returning instancetype, which are special cased in ObjC to return an instance of the receiver type (ie, if Child overrides a method on Base that returns instancetype, the method on Base returns Base while the method on Child returns Child), and can't be cleanly represented in Dart's inheritance semantics. So whenever we see a method returning instancetype, we have to copy it to all the subtypes, and from a category to its parent interface.

Before this PR there was a bug where that copying logic could bypass the usual filters. If a category was filtered out, we would still copy its instancetype methods to the parent interface. I had ignored it as an edge case, but I bumped into it again during the isIncluded PR, and it turns out to be fairly easy to fix now.

Basically, we just track a method's originCategory, then in apply_config_filterse if the origin category is filtered then we filter out the method.

To avoid making breaking changes to package:objective_c, I then had to add explicit include rules for all the categories that added instancetype methods to an interface.

Fixes #3546

[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
objective_c	Breaking	9.5.0	9.6.0-wip	9.6.0-wip	✔️

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[goderbauer]

LGTM

[goderbauer]

😆 Great that we can fix this now!