chore: migrate to chai@6

As suggested in this PR:

* explicitly depend on `chai` and `sinon` where needed
* wire both together using a `globals.js` file that is imported
  with the bundle => need to workaround tree-shaking so it is
  not accidentally optimized away

Author: nikku

karma.conf.js

@@ -37,7 +37,6 @@ module.exports = function(karma) {
 
     frameworks: [
       'mocha',
-      'sinon-chai',
       'webpack'
     ],
 
@@ -79,6 +78,10 @@ module.exports = function(karma) {
       mode: 'development',
       module: {
         rules: [
+          {
+            test: /test\/globals\.js$/,
+            sideEffects: true
+          },
           {
             test: /\.(css|bpmn)$/,
             type: 'asset/source'