Refactored module exports; (#5162)

* Refactored build pipeline;
Added module exports tests;
Added missing ESM export for parity with Axios factory;
Added `toFormData`, `formToJSON`, `isAxiosError`, `spread`, `isCancel`, `all` as named export to `index.d.ts`;

* Added ESM entry test;

* Updated README.md `installing` section;

* Added TypeScript importing test;
Added missed `CanceledError` & `AxiosHeaders` to `AxiosStatic` interface;

* Exclude `/test/module/` from tslint;

test/module/esm/index.js

@@ -0,0 +1,13 @@
+import assert from 'assert';
+import axios, {CanceledError, AxiosError, AxiosHeaders} from 'axios';
+
+assert.strictEqual(typeof axios, 'function');
+assert.strictEqual(typeof CanceledError, 'function');
+assert.strictEqual(typeof AxiosError, 'function');
+assert.strictEqual(typeof AxiosHeaders, 'function');
+
+assert.strictEqual(axios.CanceledError, CanceledError);
+assert.strictEqual(axios.AxiosError, AxiosError);
+assert.strictEqual(axios.AxiosHeaders, AxiosHeaders);
+
+console.log('ESM importing test passed');

test/module/esm/package.json

@@ -0,0 +1,16 @@
+{
+  "name": "esm-entrypoint-test",
+  "version": "1.0.0",
+  "description": "",
+  "main": "index.js",
+  "type": "module",
+  "scripts": {
+    "test": "npm i --no-save --no-package-lock && node index.js"
+  },
+  "keywords": [],
+  "author": "",
+  "license": "MIT",
+  "dependencies": {
+    "axios": "file:../../.."
+  }
+}