feat: add runtime and runtime-async packages

[chrispcampbell]

Fixes #198

This adds the @sdeverywhere/runtime and @sdeverywhere/runtime-async packages. These were originally developed in an internal repo under the names sde-model and sde-model-async. The only changes between the original sources and the ones here are:

• additional test coverage for ModelRunner and ModelScheduler
• addition of a terminate function on the ModelRunner interface
• addition of a gen-docs.js script that uses typedoc and typedoc-plugin-markdown to generate Markdown API docs (that are currently maintained in the repo under the docs directories for ease of access and updating)

These packages have been in use in En-ROADS and C-ROADS for over a year and a half and haven't had many changes in that time, so I consider them to be pretty stable. There may be a few API tweaks required once we start testing other models with them (for example, we'll need to fix the Outputs type to support models that use a "saveper" value other than 1), but I think this is good enough for a 0.1.0 release.

(Regarding the API docs, the internal repo used a different typedoc plugin that generated fancier HTML docs, but I like the simplicity of generating Markdown, and while I normally don't like keeping generated files in a repo, in this case it works pretty well since the Markdown pages are linked and browsable on GitHub, and it's easier than deploying docs to a separate server or GH Pages. We also have the added benefit of being able to see easily when there are API changes by looking at the diffs of the generated docs.)

/cc @ToddFincannon