fix: transpile to es2017 as esnext may result in unsupported JS code (#…

…593)

Transpile to es2017 to ensure compatiblity with Node.Js 8. Otherwise use of
e.g. the optional chaining operator ?. supported since typescript 3.7 results
in js not running on nodejs 8. Es2017 is the minimum to get native await support.

packages/tsconfig.base.json

@@ -13,7 +13,7 @@
     "sourceMap": true,
     "strict": true,
     "strictNullChecks": true,
-    "target": "esnext"
+    "target": "es2017"
   },
   "exclude": [
     "node_modules"