feat: add ES6 modules/TypeScript compatible export (#94)

Closes #74


## Reviewer checks

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- [x] Follows the commit message policy, appropriate for next version
- [x] Has documentation updated, a DU ticket, or requires no documentation change
- [x] Includes new tests, or was unnecessary
- [x] Code is reviewed for security by: @WilcoFiers

lib/index.js

@@ -135,7 +135,9 @@ AxeBuilder.prototype.analyze = function(callback) {
   // Check if the provided `callback` uses the old argument signature (an arity of 1). If it does, provide a helpful deprecation warning.
   var isOldAPI = callback && callback.length === 1;
   if (isOldAPI) {
-    deprecate('Error must be handled as the first argument of axe.analyze. See: #83');
+    deprecate(
+      'Error must be handled as the first argument of axe.analyze. See: #83'
+    );
   }
 
   return new Promise((resolve, reject) => {
@@ -185,3 +187,6 @@ AxeBuilder.prototype.analyze = function(callback) {
 };
 
 exports = module.exports = AxeBuilder;
+
+// TypeScript/ES6 module support (see #74).
+exports.default = AxeBuilder;