feat: export mergeConfig (#5151)

Co-authored-by: Jay <jasonsaayman@gmail.com>

test/specs/api.spec.js

@@ -50,6 +50,10 @@ describe('static api', function () {
   it('should have isAxiosError properties', function () {
     expect(typeof axios.isAxiosError).toEqual('function');
   });
+
+  it('should have mergeConfig properties', function () {
+    expect(typeof axios.mergeConfig).toEqual('function');
+  });
 });
 
 describe('instance api', function () {

test/specs/instance.spec.js

@@ -23,6 +23,7 @@ describe('instance', function () {
         'spread',
         'getUri',
         'isAxiosError',
+        'mergeConfig',
         'VERSION',
         'default',
         'toFormData',