refactor: add prefer-const to .eslintrc + fix errors (#14880)

spec/fixtures/api/mixed-sandbox-app/main.js

@@ -15,7 +15,7 @@ let currentWindowSandboxed = false
 app.once('ready', () => {
   function testWindow (isSandboxed, callback) {
     currentWindowSandboxed = isSandboxed
-    let currentWindow = new BrowserWindow({
+    const currentWindow = new BrowserWindow({
       show: false,
       webPreferences: {
         preload: path.join(__dirname, 'electron-app-mixed-sandbox-preload.js'),

spec/fixtures/api/relaunch/main.js

@@ -8,7 +8,7 @@ process.on('uncaughtException', () => {
 })
 
 app.once('ready', () => {
-  let lastArg = process.argv[process.argv.length - 1]
+  const lastArg = process.argv[process.argv.length - 1]
   const client = net.connect(socketPath)
   client.once('connect', () => {
     client.end(String(lastArg === '--second'))

spec/fixtures/workers/shared_worker_node.js

@@ -1,5 +1,5 @@
 self.onconnect = function (event) {
-  let port = event.ports[0]
+  const port = event.ports[0]
   port.start()
   port.postMessage([typeof process, typeof setImmediate, typeof global, typeof Buffer].join(' '))
 }