fix: border side getters now account for implicit borders

[veeceey]

Summary

• Fixes the inconsistency where GetBorderTop(), GetBorderRight(), GetBorderBottom(), GetBorderLeft(), and GetBorder() returned false when a border style was set via BorderStyle() without explicitly enabling sides, even though the renderer treats all sides as enabled in this case.
• The boolean getters now fall through to implicitBorders() — the same mechanism already used by the size getters (GetBorderTopSize, etc.) and the renderer (applyBorder) — making all three consistent.
• Adds comprehensive tests covering implicit borders, explicit sides, no borders, frame size consistency, and the interaction between BorderStyle() and explicit side setters.

Fixes #522

Reproduction (before fix)

s := lipgloss.NewStyle().BorderStyle(lipgloss.NormalBorder())
fmt.Println(s.GetBorderLeft())            // false (wrong — border WILL render)
fmt.Println(s.GetHorizontalBorderSize())  // 2     (correct)
fmt.Println(s.GetHorizontalFrameSize())   // 2     (correct)

After this fix, GetBorderLeft() correctly returns true.

Test plan

• All existing tests pass (including updated TestStyleUnset which was testing for the inconsistent behavior)
• New TestGetBorderSidesWithImplicitBorders covers five scenarios:
  • BorderStyle() only (implicit borders) — all getters return true
  • Border() with explicit sides — only enabled sides return true
  • No border set — all getters return false
  • Frame size consistency with BorderStyle() + padding
  • BorderStyle() then explicit side setter disables implicit for other sides
• go test ./... passes all packages (lipgloss, list, table, tree)

[veeceey]

Friendly ping - any chance someone could take a look at this when they get a chance? Happy to make any changes if needed.