Fix both GetHorizontalBorderSize and GetVerticalBorderSize

[nervo]

In both GetHorizontalBorderSize and GetVerticalBorderSize, computing size directly using getBorderStyle() can not take into accounts that some borders ("top", maybe "right", "bottom", whatever...) could have been disabled, leading to incorrect results.

This is well done in each individual GetBorder[Top|Right|Bottom|Left]Sizelike this:

lipgloss/get.go

Lines 303 to 305 in df8b3fa

	if !s.getAsBool(borderTopKey, false) {
	return 0
	}

So this pull request just make use of this correct behavior :)

[maaslalani]

Thanks so much @nervo! We really appreciate the PRs!