Add types declarations

[ker0x]

This PR add types declaration when possible. It also adds Rector as a dev dependency to facilitate code modernization.

[norkunas]

Thanks. Before merging this we should wait for geocoder-php/Geocoder#1184

Just a question about rector: if the code is migrated once, then what's the point of introducing it as dependency?

[ker0x]

@norkunas It can help with further modernization (symfony migration, code refactoring, etc.). But if you prefer, I can remove it

[norkunas]

I understand, but how often it will be needed here? :) I think rector better suits for php projects than libraries. Imho it's easy enough to run once make a PR without including it as a dependency just to be downloaded everytime in CI.

[ker0x]

@norkunas good point. I updated my PR and removed Rector from it 😉