(v2) consistency and best practices

[meowgorithm]

This is a set of breaking changes to enforce consistency and best practices in Bubbles v2.

The two big package-wide ones are:

1. All bubbles now use getters and setters for width and height. This will prevent us from having to make breaking changes in the future when changing size needs to affect other parameters.

// Before
type Model struct {
    Height int
}

// After
func (m Model) Height() int
func (m *Model) SetHeight(int)

2. Default keymaps are no longer global. This guards against the default keycap being altered on the package level.

// Before
var DefaultKeyMap = KeyMap{/* ... */}

// After
func DefaultKeyMap() KeyMap

There are also a series of small package-level adjustments which we'll outline in the release notes.