fix: border size getters when implicit borders are present

[meowgorithm]

When a border style is set and no borders have been explicitly enabled or disabled, a border is implicitly applied to all sides of the block. Prior to this fix border size getters would incorrectly return 0 when an implicit border is set. This revision corrects that behavior.

In other words:

s := lipgloss.NewStyle.Border(lipgloss.NormalBorder)

// Before
fmt.Println(s.GetBorderLeftSize()) // 0

// After
fmt.Println(s.GetBorderLeftSize()) // 1

[meowgorithm]

@bashbunni Just a heads up about this one. Technically it's a behavioral change but I think it is the right approach here. I plan on merging it today.

I encountered the need for the fix in charmbracelet/bubbletea#1204.

[bashbunni]

Hey @meowgorithm just a heads up, this is super similar to another open PR https://github.com/charmbracelet/lipgloss/pull/282/files

The main difference being naming + I like your GetBorderXSize() implementations better.