Support ES Modules by making Nylas SDK a dual package (#477)

This PR makes the Nylas SDK a dual package supporting both CommonJS and ES Module syntax. Now, the SDK's distribution package will include 3 directories:

- lib/cjs: for CommonJS
- lib/esm: for ES Modules
- lib/types: for the TypeScript declaration files

Furthermore we've also upgraded the tsconfig to target es2021 as it's the newest ECMAScript version that is fully supported by the minimum Node version supported by the SDK (v16).

.gitignore

@@ -11,3 +11,5 @@ CTAGS
 .idea
 coverage
 docs
+cjs
+esm