Extension support: Allow a field in pxt.json that will be an extension

[srietkerk]

When creating an extension, the name of the extension needs to match the repo name in order for the help pages to work. That's fine, but that means that the way the extension will appear in the extension list will be a repo name, which is usually much longer than needed. It would be nice to have a field like displayName or something similar that can be used as the extension's listed name instead of the repo name.

[sae220]

I hope the field will be added as tranaslatable.