Fix babel-loader failing on JSON5 syntax

packages/next/build/babel/loader/get-config.ts

@@ -1,4 +1,5 @@
 import { readFileSync } from 'fs'
+import JSON5 from 'next/dist/compiled/json5'
 
 import { createConfigItem, loadOptions } from 'next/dist/compiled/babel/core'
 import loadConfig from 'next/dist/compiled/babel/core-lib-config'
@@ -151,7 +152,7 @@ const isJsFile = /\.js$/
 function getCustomBabelConfig(configFilePath: string) {
   if (isJsonFile.exec(configFilePath)) {
     const babelConfigRaw = readFileSync(configFilePath, 'utf8')
-    return JSON.parse(babelConfigRaw)
+    return JSON5.parse(babelConfigRaw)
   } else if (isJsFile.exec(configFilePath)) {
     return require(configFilePath)
   }

test/integration/babel-custom/fixtures/babel-json5/.babelrc

@@ -0,0 +1,12 @@
+{
+  "presets": [
+    [
+      "next/babel",
+      {
+        "preset-env": {
+          "modules": "commonjs" // Here's a comment
+        }
+      }
+    ]
+  ]
+}

test/integration/babel-custom/fixtures/babel-json5/pages/index.js

@@ -0,0 +1 @@
+export default () => <h1>Hello World</h1>

test/integration/babel-custom/test/index.test.js

@@ -17,4 +17,8 @@ describe('Babel', () => {
   it('should allow setting targets to a string', async () => {
     await nextBuild(join(__dirname, '../fixtures/targets-string'))
   })
+
+  it('should allow babelrc JSON5 syntax', async () => {
+    await nextBuild(join(__dirname, '../fixtures/babel-json5'))
+  })
 })