feat(button): add button inner part

[DaveDev13]

A small edit that will get rid of unnecessary code and give access to the inside of the button for styling

To avoid unnecessary code, as in example #29935

Issue number: resolves #29935

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What is the current behavior?

Currently, there is no access to .button-inner, which means there is no way to change the behavior of justify-content, align-items or flex

What is the new behavior?

Adds access to the element in css via ::part(inner)

Does this introduce a breaking change?

• [ ] Yes
• [x] No