build: enable "esnext"

Problem:
Can't use newer JS features like `Array.at()`

Solution:
Set `lib=esnext`. This is still compatible with older node.js (our CI
currently tests node.js 14), because `target=es2015`. So typescript will
polyfill to es2015, but we can use esnext featuers in our `.ts` code.

tsconfig.json

@@ -3,7 +3,7 @@
     "allowSyntheticDefaultImports": true,
     "declaration": true,
     "esModuleInterop": true,
-    "lib": ["es2015", "dom"],
+    "lib": ["esnext", "dom"],
     "module": "commonjs",
     "moduleResolution": "node",
     "sourceMap": false,