Pin version of shadcn, fixes #309 (#330)

The latest version of the UI package does not include the CLI anymore,
it's published under `shadcn` now - this PR fixes that by pinning the
version to `0.8.0`, the latest previous version that works.

It would be nice to start using the new CLI, but it's incompatible with
the current setup inside NX: it expects package.json and components.json
to be defined within the same directory. I think the better approach
would be to implement a NX-specific version of the `add` command instead
of using the upstream CLI - it's [not particularly
complex](https://github.com/shadcn-ui/ui/blob/main/packages/cli/src/commands/add.ts).

packages/shadcn-ui/src/executors/add/add.impl.ts

@@ -14,7 +14,7 @@ export async function addExecutor(
 
   return execPackageManagerCommand(
     buildCommand([
-      'shadcn-ui@latest add',
+      'shadcn-ui@0.8.0 add',
       (options.component ?? '').length === 0 ? '--all' : options.component,
       options.overwrite && '--overwrite',
       '--path=src',