Fix regression in ESM plugin loading

[marcalexiei]

Fixes #656

To prevent the reintroduction of #615,
I refined the condition in package.json to return the default export only if it is the only available export.
Otherwise, the module will be returned as-is, which should resolve #656.

---

Note

Since this package supports relatively recent versions of Node:

eslint-doc-generator/package.json

Lines 95 to 97 in cc4d7d6

	"engines": {
	"node": "^18.18.0 || ^20.9.0 || >=22.0.0"
	},

the entire function could potentially be refactored in the future to primarily use an dynamic import instead of require.
For reference: Node's import support status.

Let me know if it's something you're interested in exploring!

[bmish]

Thanks.

Ideally we can add some tests for this in the near future, although I know it's kind of annoying to test.

[bmish]

@marcalexiei yes I'd love if there was a way to simplify this logic or make it more robust. Loading the plugin is quite painful right now. Feel free to experiment with improvements.