Adds basic sequence diagrams behind a layout flag

[ejulio-ts]

Adds d2sequence layout for sequence diagrams with basic functionality.
The layout is behind a flag for now as it will need changes in D2 for full support. So, to merge these changes in small PRs, lets have it behind a flag that also allows us to test it locally without much friction.

Assumptions:

• edges are kept in order as they are defined
• object order is identified as they appear in edge order
• the lifeline edge follows the shape stroke style, except for stroke-dash that is constant

No changelog as this is not ready yet. Will add the line once we got it properly integrated.

here is an example diagram

alice: "Alice\nline\nbreaker" {
    shape: person
    style.stroke: red
}
bob: "Bob" {
    shape: person
    style.stroke-width: 5
}
db: {
    shape: cylinder
}
queue: {
    shape: queue
}
service: "an\nodd\nservice\nwith\na\nname\nin\nmultiple lines"

alice -> bob: "Authentication Request"
bob -> service: "make request"
service -> db: "validate credentials"
db -> service: {
    style.stroke-dash: 4
}
service -> bob: {
    style.stroke-dash: 4
}
bob -> alice: "Authentication Response"
alice -> bob: "Another authentication Request"
bob -> queue: "do it later"
queue -> bob: "stored" {
    style.stroke-dash: 3
    style.stroke-width: 5
    style.stroke: green
}
bob -> alice: "Another authentication Response"

Updates #99

[alixander]

let's introduce sequence-diagram-specific terminology, starting with s/object/actor

[ejulio-ts]