feat: add custom icon to buttons

[gion]

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[中文版模板 / Chinese template]

This is a ...

• New feature
• Bug fix
• Site / document update
• Component style update
• TypeScript definition update
• Refactoring
• Code style optimization
• Branch merge
• Other (about what?)

I'm sorry, I'm not sure if it is actually a new blown feature, as this feels more of a refactoring.

What's the background?

1. Describe the source of requirement.

Currently, if you use a custom icon, there's no way you can integrate that icon into other components (such as buttons).

In the React version Antd, a button can simply pass a different component as an icon.
For example, if you look at the docs, you could write a button with a custom icon this way:
<Button icon={<MyCustomIcon />} />
In Vue, this is not possible.

2. Resolve what problem.

Refactor the button component to allow passing a custom icon component through slots:

<Button>
    <MyCustomIcon slot="icon" />
</Button>

3. Related issue link.

API Realization (Optional if not new feature)

1. Basic thought of solution and other optional proposal.
2. List final API realization and usage sample.
3. GIF or snapshot should be provided if includes UI/interactive modification.

What's the effect? (Optional if not new feature)

1. Does this PR affect user? Which part will be affected?
2. What will say in changelog?
3. Does this PR contains potential break change or other risk?

Changelog description (Optional if not new feature)

1. English description.

Add custom icon to buttons through slots

2. Chinese description (optional)

Self Check before Merge

• Doc is updated/provided or not needed
• Demo is updated/provided or not needed
• TypeScript definition is updated/provided or not needed
• Changelog is provided or not needed

Additional Plan? (Optional if not new feature)

If this PR related with other PR or following info. You can type here.

[github-actions]

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