Test types (#4140)

* Distinguish request and response data types

* Fix Axios headers type

`axios.headers` is not of the same type as `request.headers`, so a new type
`AxiosDefaults` was introduced

* Replace grunt-ts with dtslint

This asserts that the type definitions are valid in the specified TypeScript
version and above. This is the same tool that is used by DefinitelyTyped.

* Remove grunt-ts

* Restore typescript dependency

* Fix missing semicolons

Co-authored-by: Claas Augner <github@caugner.de>
Co-authored-by: Jay <jasonsaayman@gmail.com>

tslint.json

@@ -0,0 +1,6 @@
+{ 
+  "extends": "dtslint/dtslint.json",
+  "rules": {
+    "no-unnecessary-generics": false
+  }
+}