Fix type of the plugin object's configs field

[rhysd]

Fix #392

By this change plugin.d.ts looks like the following. configs field is no longer optional.

import type { ESLint, Linter } from 'eslint';
export type MochaPlugin = ESLint.Plugin & {
    configs: {
        all: Linter.Config;
        recommended: Linter.Config;
    };
};
declare const mochaPlugin: MochaPlugin;
export default mochaPlugin;
//# sourceMappingURL=plugin.d.ts.map

[rhysd]

This hack is ugly but I could not find any other workaround for those cyclic references.

[mdmower-csnw]

@rhysd - Should be able to close this pull request since #400 was accepted.