Upgrade to latest babel

There has been a lot of renaming and restructuring in babel, so we need
to modify our code to handle the latest version. We also need to adjust
the way we build our babel worker as babel itself no longer runs in older
browsers such as Internet Explorer.

kasmweb/utils/use_require.js

@@ -4,7 +4,7 @@ const path = require('path');
 const program = require('commander');
 const fs = require('fs');
 const fse = require('fs-extra');
-const babel = require('babel-core');
+const babel = require('@babel/core');
 
 const SUPPORTED_FORMATS = new Set(['amd', 'commonjs', 'systemjs', 'umd']);
 
@@ -150,8 +150,12 @@ function make_lib_files(import_format, source_maps, with_app_dir, only_legacy) {
 
     // NB: we need to make a copy of babel_opts, since babel sets some defaults on it
     const babel_opts = () => ({
-        plugins: [`transform-es2015-modules-${import_format}`],
-        presets: ['es2015'],
+        plugins: [],
+        presets: [
+            [ '@babel/preset-env',
+              { targets: 'ie >= 11',
+                modules: import_format } ]
+        ],
         ast: false,
         sourceMaps: source_maps,
     });