Admin Bar: Add custom property for responsive height value

[ryelle]

Sets up a new custom property --wp-admin--admin-bar--height, and uses it in all the places where the height (value) is used.

If we don't want to change all the CSS in admin-bar.css, we could also just use 7ab4b04 & 716d654 - add the custom property and use it in the default frontend code.

Themes can use this in their CSS to offset content, and it will inherit the correct size. For example:

.some-banner {
    top: var(--wp-admin--admin-bar--height);
}

It would be nice to set this to 0 when there is no admin bar, but the property needs to be on html and the admin-bar class is on body.

Trac ticket: https://core.trac.wordpress.org/ticket/52623

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