fix: add type tests

[fasttime]

Because of type inference issues, only the languages property is currently included in the definitions for plugin:

/** @type {Plugin & { languages: Record<string,Language>}} */
declare const plugin: Plugin & {
    languages: Record<string, Language>;
};

from https://unpkg.com/@eslint/markdown@6.2.1/dist/esm/index.d.ts

This PR refactors the plugin definition to ensure that all relevant properties of language plugins are included in the types. I also added type tests similar to eslint/json#61.

[fasttime]

Using variables instead of plugin.configs.recommended[0] and plugin.configs.processor[0] so we don't have to care if the order/structure of those config arrays changes.

[nzakas]

LGTM. Thanks.