Added '{d}' 'dimension'/tile size placeholder support to TileLayer.urlTemplate

[JaffaKetchup]

Fixes #1664.

Will only be breaking for users already using '{d}' normally in their URL: nobody.

[bramp]

To catch the issue with users already using {d}, you could warn if additionalOptions contains keys x/y/z/s/d and that it would override what's provided in TileCoordinates/TileLayer. Or maybe that's a support model.

[JaffaKetchup]

I don't think anyone should be using '{d}'. It's very undescriptive as a custom placeholder, does not refer to anything that that is normally used for, such as filling API keys or style params. It's also not valid in a URL outside of this.

[bramp]

While I don't disagree that folks should not be using, Hyrum’s Law states otherwise :). Just trying to avoid someone being confused in future.

[JaffaKetchup]

I'll add it to the Migration instructions out of an abundance of caution! 😂

[JaffaKetchup]

I'm going to bypass normal procedures here, and merge without a second maintainer's approval. This is a very small change, which adds no really new functionality or ideas. v6 needs to get moving at some point!

Happy to make changes if a second maintainer requests it.