update .swcrc configuration

The `env` and `jsc.target` combination isn't valid.
Removing `env` works, but it introduces a lot of additional code that
wasn't there before.
I started by using es2015, es2016 and last but not least es2018. es2018
produces the most similar code compared to before. The main differences
are:
- Single vs double qoutes
- Now we will have `catch (e) {}` instead of the more modern `catch {}`

.swcrc

@@ -2,12 +2,8 @@
   "module": {
     "type": "commonjs"
   },
-  "env": {
-    "targets": {
-      "node": "12.13.0"
-    }
-  },
   "jsc": {
+    "target": "es2018",
     "parser": {
       "syntax": "typescript",
       "tsx": false