Support browsers from before 2020 (#4579)

* Support browsers from before 2020

As reported in #2825, #2826 and #3051, almost everything works in older browsers. This setting here prevented me from updating and I think it is an obvious enhancement to not restrict to browsers from 2020+. There should not be any measurable downsides of this change since es6 and es2020 are pretty similar with only minor differences.

* Include lib (polyfills) for <es2020 targets

* Assume all modern dom features despite es6 syntax

* Add modern dom iterators to es6 environment

Co-authored-by: Joe Previte <jjprevite@gmail.com>
Co-authored-by: Asher <ash@coder.com>

tsconfig.json

@@ -1,6 +1,7 @@
 {
   "compilerOptions": {
-    "target": "es2020",
+    "target": "es6",
+    "lib": ["es2020", "dom", "dom.iterable"],
     "module": "commonjs",
     "moduleResolution": "node",
     "strict": true,