Dependency license tracking with FOSSA

[wipfli]

I saw that the iOS platform has some dependency license tracking using FOSSA. At least the README says:

The licenses of its dependencies are tracked via FOSSA:

Should we implement something like that again?

[wipfli]

We might get enough with the free tier at https://fossa.com/pricing

[wipfli]

The number of licenses in Native is a bit confusing for me:

./platform/android/LICENSE.md
./platform/ios/LICENSE.md
./LICENSE.md
./LICENSE.mbgl-core.md