Add TypeScript target and module settings explainer

[tom-sherman]

This MR adds an explainer to the settings you should set in tsconfig.json lest you run into type errors.

See #675

[marvinhagemeister]

Awesome, thank you so much for your PR 🙌

[katywings]

@tom-sherman I gotta update my project template then which atm just sets tsconfig target: es5 :).

But if I understand it correctly the tsconfig does not have effects upon the microbundle js output right?

Out of curiosity: which inconsistent type errors did you get without setting target/module to ESNext?

[tom-sherman]

@katywings Yeah microbundle handles the output so this setting is there just for typechecking via tsc and also VSCode.

My type errors were caused by differences in the DOM lib types between ES5 and ESNext target versions, specifically the URLSearchParams interface