Implement auto-discovery of plugins

[illright]

When the config file isn't present, the linter should auto-detect what plugin is installed by searching package.json that contains it for steiger-plugin-*, @*/steiger-plugin, or @*/steiger-plugin-*, as ESLint suggests in its docs for plugin creators:

This will allow us to stop defaulting to the FSD ruleset when no config is present, and as such, remove the dependency from steiger on @feature-sliced/steiger-plugin