fixes#6024.
### Problem
- `getPlugin()` defaults to `UnknownPlugin`, so methods like `openModal`
are not visible , since core is not aware of that plugin type
### Proposed change
- Introduce a types-only registry in core:
- `export interface PluginTypeRegistry<M extends Meta, B extends Body>
{}`
- Overload `getPlugin` to return a precise type when the id is a known
key of the registry.
- add `Dashboard` to PluginTypeRegistry through module augmentation:
- `'Dashboard': Dashboard<M, B>`.
- When a project imports from `@uppy/dashboard`, its module augmentation
extends PluginTypeRegistry, adding the correct type into it
- I've added Tests , kept them in a separate file so it's easier to
review , once this approach gets approved I'll add them to
`Uppy.test.ts`
Once this PR gets a positive review I'll add this for other plugins ,
currently only added for `@uppy/dashboard`
**Build with Local tarball can be checked here**
https://stackblitz.com/~/github.com/qxprakash/uppy-type-test?file=type_test.ts
79e6460a6c