Refactor @getodk/webform 2/2

[sadiqkhoja]

From #164 (comment)

• When using provide/inject, would be nice to establish a pattern where they use normal references so the connection between provided/injected state can be followed in editor, and types can be defined in provide. I did a quick spike on this not long ago and it seemed possible, happy to dig up the stash if there’s interest.
• Events make me nervous for reasons similar to provide/inject. They seem a bit easier to follow at component boundaries, but the lack of clear types with a single source of truth is still concerning.
• The tooling boundary between .vue and .ts modules is a continuing headache. Common case: TS/ESLint differing on whether a type is any. Case that stood out here: props interface exported from component, referenced in test. Would like to consider other approaches that minimize this. And/or set aside some time to see if any tooling updates address these shortcomings.