@endo/eslint-plugin: provide "flat" recommended config

[boneskull]

What is the Problem Being Solved?

@endo/eslint-plugin provides a recommended configuration, but it is only for the legacy (pre-ESLint v9) config format.

Description of the Design

Follow this guide, I guess.

Security Considerations

none

Scaling Considerations

none

Test Plan

none

Compatibility Considerations

This may require @jessie.js/eslint-plugin to be upgraded as well. I don't know if that plugin is compatible with ESLint v9.

Upgrade Considerations

Should be backwards-compatible with ESLint v8 consumers.

(cc @michaelfig @erights re: jessie plugin)