(v2) feat: add compat package

[aymanbagabas]

This adds a compatibility layer for Lip Gloss that provides a way to continue using AdaptiveColor and CompleteColor. It's impure because it uses global variables, is not thread-safe, and only works with the default standard I/O streams.

Users are encouraged to use the pure API when possible.

Here's an example to use a different output as the default:

import (
	"os"

	"github.com/charmbracelet/colorprofile"
	"github.com/charmbracelet/lipgloss/v2/compat"
)

func init() {
	compat.Profile = colorprofile.Detect(os.Stderr, os.Environ())
	compat.HasDarkBackground, _ = lipgloss.HasDarkBackground(os.Stdin, os.Stderr)
}