Feature: ability to import d2 files and reuse common components

[shikharid]

Ability to import d2 files inside another and ways to inherit styling will be a very useful feature for large scale projects.

Something like this should make:

• maintaining these diagrams easier
• teams will be able build a base set of components with custom styling, common lingo and shapes and reuse them

Also conforms to how software teams operate, in a declarative way.
Some leads create a set of guidelines on how to document stuff and the team follows it.

We will need some syntax for defining shape styling (which are not to be rendered) and ability to inherit/override them in nodes

Something like:

microservice-base.d2

d2-templates: {
  microservice-server: {
    shape: rectangle
    style: {
      stroke: "#53C0D8"
      stroke-width: 5
      shadow: true
    }
  }
  
  microservice-db: {
    shape: cylinder
    style: {
      stroke: "#53C0D8"
      stroke-width: 5
      shadow: true
    }
  }
}

my-monolith.d2

import: ./microservice-base.d2

my-monolith: {
  shape: microservice-server
  my-monolith-db {
    shape: shape-microservice-db
  }
}

[iAdanos]

+1, since it's the painkiller for complex diagrams

[iAdanos]

I think it's related to #554