Automate generation of lists of package:objective_c exports

[liamappelbe]

package:objective_c exports a bunch of generated classes/enums etc. The lists of these exports are duplicated in a few different places. We have a test that verifies all the lists match, but it would be better to automatically generate them. The tests uses regexps to find all the important things in the generated file, so we could instead use those regexps to generate the lists directly in pkgs/objective_c/tool/generate_code.dart.

Relevant files:

• pkgs/objective_c/lib/objective_c.dart
• pkgs/objective_c/ffigen_objc.yaml
• pkgs/objective_c/test/interface_lists_test.dart
• pkgs/ffigen/lib/src/code_generator/objc_built_in_functions.dart

[liamappelbe]

Btw pkgs/objective_c/ffigen_objc.yaml probably doesn't need to explicitly list all the interfaces etc