Update to Preact X

[zaninime]

Description

Update from Preact 8.x to Preact 10. While doing so, remove the out-of-tree dependency on preact-context (now available upstream).

Packages

List all packages that have been changed.

• preact-fela
• example-preact

Versioning

Major, as this is not backwards-compatible.

Checklist

Quality Assurance

You can also run yarn run check to run all 4 commands at once.

• The code was formatted using Prettier (yarn run format)
• The code has no linting errors (yarn run lint)
• All tests are passing (yarn run test)
• There are no flow-type errors (yarn run flow)

Changes

If one of the following checks doesn't make sense due to the type of PR, just check them.

• Tests have been added/updated
• Documentation has been added/updated
• My changes have proper flow-types