fix(theme): respect custom tag prop in VPButton component

[rileychh]

Description

This PR fixes a bug in the VPButton component of the default VitePress theme. Previously, the tag prop was not being properly respected when determining the component to render. This change ensures that the tag prop takes precedence, falling back to a or button based on the presence of the href prop only if no custom tag is specified.

Here's a simple script that demonstrates the effect of this change:

const props = {
  tag: 'div',
  href: undefined,
}

// Before: Incorrectly evaluates to 'a'
const component = props.tag || props.href ? 'a' : 'button'

// After: Correctly evaluates to 'div'
const fixed = props.tag || (props.href ? 'a' : 'button')

In this example, even though we specify a custom tag 'div' and there's no href, the original code would incorrectly choose 'a'. The fixed version correctly respects the custom tag 'div'.

Linked Issues

None.

Additional Context

None.

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