Fallback to kernelspec to check if it's a Python notebook (#12875)

## Summary

This PR adds a fallback logic for `is_python_notebook` to check the
`kernelspec.language` field.

Reference implementation in VS Code:
1c31e75898/extensions/ipynb/src/deserializers.ts (L20-L22)

It's also required for the kernel to provide the `language` they're
implementing based on
https://jupyter-client.readthedocs.io/en/stable/kernels.html#kernel-specs
reference although that's for the `kernel.json` file but is also
included in the notebook metadata.

Closes: #12281

## Test Plan

Add a test case for `is_python_notebook` and include the test notebook
for round trip validation.

The test notebook contains two cells, one is JavaScript (denoted via the
`vscode.languageId` metadata) and the other is Python (no metadata). The
notebook metadata only contains `kernelspec` and the `language_info` is
absent.

I also verified that this is a valid notebook by opening it in Jupyter
Lab, VS Code and using `nbformat` validator.

crates/ruff_notebook/resources/test/fixtures/jupyter/kernelspec_language.ipynb

@@ -0,0 +1,48 @@
+{
+ "cells": [
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "# Kernel spec language\n",
+    "\n",
+    "This is a test notebook for validating the fallback logic of `is_python_notebook` to check `kernelspec.language` if `language_info` is absent.\n"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "vscode": {
+     "languageId": "javascript"
+    }
+   },
+   "outputs": [],
+   "source": [
+    "function add(x, y) {\n",
+    "    return x + y;\n",
+    "}"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "import os\n",
+    "\n",
+    "print(\"hello world\")"
+   ]
+  }
+ ],
+ "metadata": {
+  "kernelspec": {
+   "display_name": "Python 3",
+   "language": "python",
+   "name": "python"
+  }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 2
+}