Intent to begin work on Indexed Database API 3.0

Indexed DB 2.0 hit Rec status in January [1].

The current editors (Ali Alabbas from Microsoft, Joshua Bell from Google)
intend to work on the next iteration of the specification, "3.0",
continuing the effort within the Web Applications WG. As with 2.0 we plan
to iterate rapidly on the ED, make WDs snapshots when notable changes
appear, and aim to close off 3.0 for normative additions in roughly a year,
hoping to hit rec in about 2 years. I'd expect a FPWD in the 3-6 month
timeframe.

The highest priority items are ensuring that we have a new ED that reflects
integration with other standards (e.g. incorporating outstanding PRs [2]),
editorial changes to clarify the text and some minor functional additions
where we have achieved consensus (see issues [3]).

The big ticket items that will likely fall into the "3.0" window include
attempting to modernize the API to make it more approachable and ergonomic,
and a change observation API.

[1] https://www.w3.org/blog/news/archives/6795
[2] https://github.com/w3c/IndexedDB/pulls
[3] https://github.com/w3c/IndexedDB/issues