Help user determine region by lookup against the eBird API

[synrg]

Rather than have the user carry around eBird region codes in their head, it would be good to provide a way that they can lookup regions using the API.

• Given that an exhaustive list of regions is not very useful, provide commands to browse down to a reasonable subset of regions to choose from.
• Also, assuming the regions don't change very often, cache the result for a fairly long time (a day, perhaps) to avoid unnecessary API calls.

e.g.

[p]ebird regions <region>

Where the help would describe some common region values, and [p]ebird regions would list regions that are direct descendants of that region (but not recurse any further), paging output if necessary.

[synrg]

Realistically, so long as the eBird Discord community isn't driving this, there will not be much more work on eBird support in general. Though I have an eBird account, I do not yet use the platform personally, nor really know the community well enough to be able to prioritize building out more cog functionality to meet their needs. So I'm seeking collaboration with developers and non-developers alike with ties to the eBird community before proceeding.