dynamic import of whatwg-fetch to allow environments without it

[BruceMacD]

Got over-zealous on the last change to make fetch a dynamic import. This change allows for the same functionality as #125 but it keeps whatwg-fetch rather than replacing it with node-fetch. This preserves current behavior.

[kwaa]

Maybe also make whatwg-fetch an optional dependency to prevent users who don't need it from installing it?

# package.json
{
- "dependencies": {
-   "whatwg-fetch": "^3.6.20"
- }
+ "peerDependencies": {
+   "whatwg-fetch": "^3.6.20"
+ },
+ "peerDependenciesMeta": {
+   "whatwg-fetch": {
+     "optional": true
+   }
+ }
}

https://caniuse.com/fetch