feat: support new config system

[jjangga0214]

Supporting eslint's new config system of eslint.

The legacy config system always has require()d plugins and sharable configs, while the new system is available in both ESM and CJS.

Therefore conditional export is great to keep compatibility while introducing the new config.

plugin: protocol(e.g. plugin:markdown/recommended) is not valid any more. In the new config system, a bi-directional dependency between a plugin object and a preset config object is detached. From now on, only a config object should depend on a plugin object, not vice-versa. Thus, the new plugin (a new entry point index.js) doesn't have configs property, following the spec, though having the property does not mean incompatibility.

Users should have a way to import shareable config independently.
Hence eslint-plugin-markdown/recommended is introduced as a new entry point for it.

[linux-foundation-easycla]

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[jjangga0214]

I also wrote README. And I've taken care to make sure this PR is not a breaking change.

[jjangga0214]

@nzakas May I ping?
I want to get feedback from you about the design in this PR, as I also want to apply similar changes to other plugins.

[kecrily]

Can you add a unit test for new config system?

[jjangga0214]

@kecrily Maybe later?
Or I expect whoever is available can do that before I do.

[nzakas]

Superseded by #232