Remove the stdlib and inspector dependency from the runtime

[mbostock]

We’ve implemented our own self-hosted standard library within Framework, but we’re still using the same Runtime that notebooks use which depends on the notebook standard library. Related to #573, we should restructure the Runtime so that it doesn’t depend on @observablehq/stdlib and @observablehq/inspector, giving users (and us) more flexibility and saving a few bytes.

[mbostock]

I tree-shaked the runtime dependency in #843, so this followup task just applies to releasing a new version of the Observable Runtime that does the tree-shaking internally (rather than bundling stdlib). See this workaround:

https://github.com/observablehq/framework/blob/main/src/client/runtime.js

(I’m still including the inspector because we need that.)