Add a placeholder prop to the Avatar component

[UnderHear]

Feature request

Add an optional placeholder prop to Avatar so it can display text, such as an initial or short label, when no image URL is provided.

For example:

<Avatar placeholder="S" size={24} />
<Avatar placeholder="M" size={32} />
<Avatar placeholder="L" size={40} />
<Avatar placeholder="XL" size={56} />
<Avatar placeholder="XXL" size={72} />

The placeholder should be centered inside the avatar and scale appropriately with size. It should follow the existing circular or square shape. When src is provided, the image should take precedence.

Motivation

The current Avatar renders an <img> and requires src, so there is no built-in way to show a text avatar for a user who has no profile image. A placeholder prop would let applications use the same component and sizing API for both image and text avatars, as shown in the attached example.