Merge cli-plugin-api into cli-node

Move createCliPlugin and related types from the standalone
@backstage/cli-plugin-api package into @backstage/cli-node and
remove the now-empty package.

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
Made-with: Cursor

packages/cli/package.json

@@ -50,7 +50,6 @@
     "@backstage/catalog-model": "workspace:^",
     "@backstage/cli-common": "workspace:^",
     "@backstage/cli-node": "workspace:^",
-    "@backstage/cli-plugin-api": "workspace:^",
     "@backstage/config": "workspace:^",
     "@backstage/config-loader": "workspace:^",
     "@backstage/errors": "workspace:^",