Fixes: #1273

Context

When looking at our documentation, the very first TypeScript example shows: https://lit.dev/docs/components/properties/

class MyElement extends LitElement {
  @property()
  name: string;
}

This results in a TypeScript error: Property 'name' has no initializer and is not definitely assigned in the constructor..

This can be globally resolved by relaxing the TypeScript compiler via "strictPropertyInitialization": false, but we should prefer documentation that is as explicit/strict as possible.

We have an example in the playground where it makes sense to not initialize a property. So I've added that syntax to our properties.md docs.

Fix:

class MyElement extends LitElement {
  @property()
  name?: string;
}

This clearly states that the property should be a string, but it can also be undefined if no-one initialized it.

Risk & Tests

This is a low risk content only change – tested by checking out preview URL.