Refactor semagrams initialization code

Currently, Semagrams calls the render function inside its intialization code:

https://github.com/AlgebraicJulia/Semagrams.jl/blob/01b51a602e5fcf309497ee7d9e77aba6cc266c57/scala/core/src/base.scala#L66-L79

It would be better to separate the code that creates the svg element into something like a "semagram widget", so that it can be inserted at arbitrary places inside of a Laminar application. In the long run, I think we may realistically want to have multiple "semagram widgets" within a more complex DOM structure.

I think it would be nice if the "apps" were in control of calling `render` and determining what gets passed to it.

```
val sharedState: ACSet = ...
val appElement = div(
h1("Process"),
SemagramWidget(projectProcess(sharedState)),
h1("Structure"),
SemagramWidget(projectStructure(sharedState)),
)
render(appContainer, appElement)
```

The pseudo-code above is meant to illustrate a page with two embedded semagram widgets that share some common state via an `ACSet`. It's intended as a sketch, a lot of details still need to be fleshed out.