feat: add sixel to ansi package

[raphamorim]

PR originally implemented by @CannibalVox

Ref: #352

Most of this PR changes are just updates on the implementation per requests.

API

Here's what the API looks like.

import  "github.com/charmbracelet/x/ansi/sixel"

var (
    buf bytes.Buffer
    e sixel.Encoder
    img image.Image // imagine this is populated with image data
)


if err := e.Encode(&buf, img); err != nil {
	return 0, err
}

_, _ = io.WriteString(os.Stdout, ansi.SixelGraphics(0, 1, 0, buf.Bytes()))

Example

cd examples/img2term && go run . ./../../ansi/fixtures/graphics/JigokudaniMonkeyPark.png

Misc

Example while running go bench with fmt.Println iterm2/wezterm/rio:

[raphamorim]

iterm2/wezterm/rio

[meowgorithm]

I can't speak to the implementation details, but functionally speaking, LGTM