Dependencies between plugins

A plugin can declare that it needs other plugins to be active first:

export default {
  id: "notes-sync",
  name: "Notes sync",
  dependsOn: ["notes"],
  setup(ctx) {
    const notes = ctx.useApi("notes");
    if (!notes) {
      // The dependency is missing or disabled.
      return;
    }
    notes.onDocumentSaved((doc) => {
      // react to the dependency's API
    });
  },
};

Rules:

dependsOn is a list of plugin ids.
ctx.useApi(id) only works for ids listed in dependsOn. It returns undefined when the dependency is missing, disabled or exposes no API.
A plugin exposes an API with ctx.provideApi(api) during setup. Returning the API from setup() does the same thing.
Typing: ids listed in the PluginApis interface of plugin-api are typed without generics (ctx.useApi("documents") returns DocumentsApi). A plugin publishing its own API adds itself by module augmentation, in a type-only api.ts that consumers include:
```
declare module "@app/kernel/plugin-api" {
  interface PluginApis { "my-plugin": MyApi }
}
```
ctx.useApi<T>("other-id") still works for untyped ids.
If a dependency is installed but disabled, the Plugins panel offers to enable it. If it is not installed, you must install it first.
Disabling or removing a plugin disables all active plugins that depend on it, directly or transitively.