Viewport quality-of-life improvements

[meowgorithm]

This update contains a number of improvements to the viewport Bubble to make it more pleasant to work with. Keep in mind that as of this update viewport models are strongly encouraged to be initialized with the new New(width, height int) Model function, however it's currently optional to reduce friction when upgrading.

const width, height = 40, 80
vp := viewport.New(width, height)

• Viewport models now have a Style lipgloss.Style property for setting borders, margins, and padding. No more magic numbers, random constants, and awkward arithmetic!
• Keybindings are now defined via the key Bubble and can now be easily remapped.
• The mouse wheel-based scroll speed can now be customized via the model's MouseWheelDelta int property
• The mouse wheel can now be disabled via the model's MouseWheelEnabled bool property