Fix getHeadingFontSize types

[sarayourfriend]

Allows passing size="3" OR size={3}, following the ReactText convention.

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🔍 Inspect: https://vercel.com/itsjonq/g2/GKWCAKRdoYp3cg9pGhGExJuwCBg2
✅ Preview: https://g2-git-fix-getheadingfontsize-itsjonq.vercel.app

[ItsJonQ]

Interesting! Check out the snapshot diff:

    -   font-size: calc(1 * 13px);
    -   font-size: calc(1 * var(--wp-g2-font-size));
    +   font-size: calc((13 / 13) * 13px);
    +   font-size: calc((13 / 13) * var(--wp-g2-font-size));

Mathematically, I think it's still the same :).
It looks like we'd be relying more on the CSS calc, which is fine :D

[sarayourfriend]

@ItsJonQ Yeah, they're mathematically the same (it made it easier to write unit tests for 😁) I'll update those snapshots, whoops!