Adding a type guard for AxiosError (#2949)

Co-authored-by: Jason Kwok <JasonHK@users.noreply.github.com>

lib/axios.js

@@ -47,6 +47,9 @@ axios.all = function all(promises) {
 };
 axios.spread = require('./helpers/spread');
 
+// Expose isAxiosError
+axios.isAxiosError = require('./helpers/isAxiosError');
+
 module.exports = axios;
 
 // Allow use of default import syntax in TypeScript