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>

tsconfig.json

@@ -0,0 +1,14 @@
+{
+  "compilerOptions": {
+    "module": "es2015",
+    "lib": ["dom", "es2015"],
+    "types": [],
+    "moduleResolution": "node",
+    "strict": true,
+    "noEmit": true,
+    "baseUrl": ".",
+    "paths": {
+      "axios": ["."]
+    }
+  }
+}