[docs] Fix 'property' has no initializer issue in TypeScript property samples

[AndrewJakubowicz]

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.

[github-actions]

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[augustjk]

It's okay for now.. optional won't be valid once we start using accessor but we can cross that bridge later. Alternative would be providing defaults but I can't think of good ones at the moment.