Add support in eslint-config-hypothesis for eslint flat config

[acelaya]

Eslint 9 will ship with a new config format which, for v8, is supported but disabled by default.

In order to migrate hypothesis projects to the flat config, we need to migrate our plugin into that format first.

A migration guide can be found here: https://eslint.org/docs/latest/extend/plugin-migration-flat-config

Since flat config require plugins to be explicitly imported, we could initially provide another entry point for this package and make "regular" and flat configs coexist until all projects have been migrated.

[acelaya]

Completed by #50