feat: implement allowFileAccess loadExtension option (#25198)

Co-authored-by: Samuel Maddock <samuel.maddock@gmail.com>
Co-authored-by: Jeremy Rose <jeremya@chromium.org>

docs/api/extensions.md

@@ -15,7 +15,7 @@ extension capabilities.
 
 Electron only supports loading unpacked extensions (i.e., `.crx` files do not
 work). Extensions are installed per-`session`. To load an extension, call
-[`ses.loadExtension`](session.md#sesloadextensionpath):
+[`ses.loadExtension`](session.md#sesloadextensionpath-options):
 
 ```js
 const { session } = require('electron')

docs/api/session.md

@@ -750,9 +750,13 @@ will not work on non-persistent (in-memory) sessions.
 
 **Note:** On macOS and Windows 10 this word will be removed from the OS custom dictionary as well
 
-#### `ses.loadExtension(path)`
+#### `ses.loadExtension(path[, options])`
 
 * `path` String - Path to a directory containing an unpacked Chrome extension
+* `options` Object (optional)
+  * `allowFileAccess` Boolean - Whether to allow the extension to read local files over `file://`
+    protocol and inject content scripts into `file://` pages. This is required e.g. for loading
+    devtools extensions on `file://` URLs. Defaults to false.
 
 Returns `Promise<Extension>` - resolves when the extension is loaded.
 
@@ -775,7 +779,11 @@ const { app, session } = require('electron')
 const path = require('path')
 
 app.on('ready', async () => {
-  await session.defaultSession.loadExtension(path.join(__dirname, 'react-devtools'))
+  await session.defaultSession.loadExtension(
+    path.join(__dirname, 'react-devtools'),
+    // allowFileAccess is required to load the devtools extension on file:// URLs.
+    { allowFileAccess: true }
+  )
   // Note that in order to use the React DevTools extension, you'll need to
   // download and unzip a copy of the extension.
 })