Use const for data classes + migrate example

[M-Ahal]

1. Many data classes could have had const constructors but didn't. I added it for all.
   • some were also missing final params
   • Most BaseData could be const, and that was preventing extend MaterialData... and CupertinoData... from being const
   • These performance updates did percolate down to the examples
2. Removed an unimplemented todo file
3. Migrated the example to use the new build system
   • Regenerated the project and added back the lib/**, pubspec.yaml, README.md.
   • Ran build on XCode to generate any remaining files
4. Added FVM to gitignore + removed from VCS
   • Link to /User/Johnstone/.fvm/... was in VCS, so that should not be there
   • pubspec.yaml had pinned a min flutter: ">=3.29.0", but this did not reflect in the "3.24.0" in the fvm_config.json. It's better to allow a developer to use their own FVM version, and set min and max in the pubspec directly

[M-Ahal]

@aqwert is there something left or unclear. This is a relatively small PR. I'd like to fix up something if broken. The const classes are really needed.
Please lmk!