Consider a less confusing package name

[robintown]

Is your feature request related to a problem? Please describe.
The name 'matrix-widget-api' causes the package to frequently get confused with the The Widget API when talking to people about widgets

Describe the solution you'd like
Something like matrix-widget-sdk?

Describe alternatives you've considered
Unsure

Additional context
Add any other context or screenshots about the feature request here.

[turt2live]

(loose) rationale for naming: "SDK" implies everything is built in for you to make a widget, but it also has a client surface to it which makes it more like a library. "matrix-widget-lib" was awkward to type, so picked "matrix-widget-api".

[turt2live]

If we do update the name of this package, the following will also need to be updated:

• Element Web + React SDK
• Example sticker picker widget
• Pre-refactoring spec & guides (they're still referenced today)
• ... other things I forgot, but probably the downstream consumers of this outside of Element.