Use consistent resolve extensions alias for compiler (#58725)

## What?

Changes the default resolve extensions to be consistent between
server/client compilation.

Currently browser prefers `.mjs` then `.js` and the server compiler
prefers `.js` then `.mjs`.

I added that behavior back in 2018 before mjs/cjs extensions were fully
baked and many npm libraries had incorrect usage of it (PR: #5898).

In the new setup with ESM resolving in Node.js writing out the full
extension is required for `.mjs` files, meaning this "magic resolving"
of a file to `.mjs` is no longer correct. In the future we'll likely
want to remove `.mjs` from the default list of resolve extensions to
align with the Node.js ESM resolver a bit more.

For now we have to keep it in order to not break existing applications
that rely on that behavior, an example of that seems to be contentlayer.

## How?

Removed the condition and aligned it on a single list instead of
separate lists.

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packages/next/src/build/webpack-config.ts

@@ -587,9 +587,7 @@ export default async function getBaseWebpackConfig(
 
   const resolveConfig: webpack.Configuration['resolve'] = {
     // Disable .mjs for node_modules bundling
-    extensions: isNodeServer
-      ? ['.js', '.mjs', '.tsx', '.ts', '.jsx', '.json', '.wasm']
-      : ['.mjs', '.js', '.tsx', '.ts', '.jsx', '.json', '.wasm'],
+    extensions: ['.js', '.mjs', '.tsx', '.ts', '.jsx', '.json', '.wasm'],
     extensionAlias: config.experimental.extensionAlias,
     modules: [
       'node_modules',