feat(style): add @modal-header-border-color-split variable

[OzanBarsal]

This is a Component style update for Modal

What's the background?

We're using antdv as our project's component library. I was looking to change to header split border's color when I realized the less variable was missing.

@modal-footer-border-color-split exists in this project and even @modal-header-border-color-split exists in antd of React library so I thought it makes sense that it also exists here.

I haven't found any issues regarding this, and instead of opening one, I'm opening a PR that adds the variable to default.less and uses it in modal.less

What's the effect?

This change allows the customization of modal component's header's split border's color to be customized separately from other split border colors.

Has no effect unless the variable is set to a different value in a custom theme. No breaking changes.

Resolves #2196

Changelog description

Added @modal-header-border-color-split theme variable.

[tangjinzhou]

Because the style uses script to synchronize from antd. So you better submit pr to antd first.

[OzanBarsal]

Because the style uses script to synchronize from antd. So you better submit pr to antd first.

Oh that's interesting because the variable exists on antd repo
Just realized that you're syncing styles from a pretty old version of antd is there a specific reason for this?

[tangjinzhou]

I did not search @modal-header-border-color-split through github, thinking that antd has no relevant features. Hahaha

[github-actions]

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