Update rollup/plugin-commonjs to make named exports of cjs packages work

From what I remember this got added in v12

Fixes #5

packages/file-handling/package.json

@@ -4,6 +4,9 @@
   "main": "src/index.js",
   "license": "LGPL-2.1-or-later",
   "devDependencies": {
+    "@rollup/plugin-commonjs": "^12.0.0",
+    "@rollup/plugin-json": "^4.0.2",
+    "@rollup/plugin-node-resolve": "^8.1.0",
     "chai": "^4.2.0",
     "i18next-scanner": "^2.10.2",
     "karma": "^5.0.1",
@@ -13,13 +16,10 @@
     "mocha": "^7.1.1",
     "node-sass": "^4.12.0",
     "puppeteer": "^2.1.1",
-    "rollup": "^2.6.1",
-    "@rollup/plugin-commonjs": "^11.0.0",
+    "rollup": "^2.18.0",
     "rollup-plugin-consts": "^1.0.1",
     "rollup-plugin-copy": "^3.1.0",
     "rollup-plugin-delete": "^1.1.0",
-    "@rollup/plugin-json": "^4.0.2",
-    "@rollup/plugin-node-resolve": "^7.0.0",
     "rollup-plugin-serve": "^1.0.1",
     "rollup-plugin-terser": "^5.1.1",
     "vpu-common": "file:./vendor/common"

packages/file-handling/src/vpu-nextcloud-file-picker.js

@@ -5,9 +5,7 @@ import VPULitElement from 'vpu-common/vpu-lit-element';
 import {MiniSpinner} from 'vpu-common';
 import * as commonUtils from 'vpu-common/utils';
 import * as commonStyles from 'vpu-common/styles';
-// `import {createClient} from 'webdav/web';` didn't seem to work if fileupload demo page is built
-// `import createClient from 'webdav/web';` didn't seem to work if Signature project is built
-import * as webDavWeb from 'webdav/web';
+import {createClient} from 'webdav/web';
 import {classMap} from 'lit-html/directives/class-map.js';
 import {humanFileSize} from 'vpu-common/i18next';
 import Tabulator from 'tabulator-tables';
@@ -125,7 +123,7 @@ export class NextcloudFilePicker extends ScopedElementsMixin(VPULitElement) {
             const apiUrl = this.webDavUrl + "/" + data.loginName;
 
             // https://github.com/perry-mitchell/webdav-client/blob/master/API.md#module_WebDAV.createClient
-            this.webDavClient = webDavWeb.createClient(
+            this.webDavClient = createClient(
                 apiUrl,
                 {
                     username: data.loginName,