feat: Compiler should support union types for the @Element decorator

[itsmebasti]

Prerequisites

• [X] I have read the Contributing Guidelines.
• [X] I agree to follow the Code of Conduct.
• [X] I have searched for existing issues that already include this feature request, without success.

Describe the Feature Request

Right now, the ESlint Compiler throws an exception when I define a union type for my element:

@Element() element: HTMLMyComponentElement | undefined;

To satisfy my "strict": true Typescript config I have to define it like so. Pretending element! exists is not the right solution since it will be undefined until it's rendered.

Describe the Use Case

It's required when using Typescript strict mode.

Describe Preferred Solution

No response

Describe Alternatives

No response

Related Code

No response

Additional Information

No response