feat: add content part to toast

[Hurubon]

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

Add the attribute part="content" to toast-content to allow styling via css.

Describe the Use Case

Currently, toast-content cannot be styled via css. For example, the padding around it cannot be changed.

Describe Preferred Solution

No response

Describe Alternatives

No response

Related Code

No response

Additional Information

No response