Rename Dropdown to Picker

[lazd]

This came out of the V3 API discussions. Though I doubt this could be considered blocking to React, this was a big point of contention and the resolution was never implemented as it’s a breaking change.

[Patil2099]

@lazd Can I work on this issue?

[lazd]

hey @Patil2099, we're not ready to make this or other breaking changes yet.

Check out the issues here and pick one up if you like: https://github.com/adobe/spectrum-css/projects/10 Thanks!

[GarthDB]

SDS-3478