mention config requirements in tools → development → typescript

[raylu]

https://lit.dev/docs/tools/development/#typescript currently doesn't explain that you need to ensure useDefineForClassFields is off and it's pretty tricky to figure out that your problem is in tsconfig.json when this happens because there are no tsc errors and initial renders are fine (components just don't update)

#479 added the important bits but only to the middle of components → reactive properties

also, https://stackblitz.com/edit/lit-element-typescript-demo is broken in exactly this way! it targets esnext, which defaults useDefineForClassFields to true

possibly related: #583