Lint docs js too and use `strict consistently. (#24160)

build/postcss.config.js

@@ -1,3 +1,5 @@
+'use strict'
+
 module.exports = (ctx) => ({
   map: ctx.file.dirname.startsWith('docs') ? false : {
     inline: false,

build/rollup.config.js

@@ -1,3 +1,5 @@
+'use strict'
+
 const path    = require('path')
 const babel   = require('rollup-plugin-babel')
 const resolve = require('rollup-plugin-node-resolve')

build/workbox.js

@@ -1,3 +1,5 @@
+'use strict'
+
 const fs = require('fs')
 const path = require('path')
 const swBuild = require('workbox-build')