Update tsconfig target version

ES2020 should be easily safe: in theory it's fine for Node v14. We could
go further but there's limited benefit, the main thing is avoiding heavy
workarounds to syntactic features (like async/await).

tsconfig.json

@@ -1,6 +1,6 @@
 {
     "compilerOptions": {
-        "target": "es6",
+        "target": "es2020",
         "module": "commonjs",
         "outDir": "./dist",
         "declaration": true,