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-internal/package.json

@@ -22,7 +22,7 @@
     "test": "backstage-cli package test"
   },
   "dependencies": {
-    "@backstage/cli-plugin-api": "workspace:^"
+    "@backstage/cli-node": "workspace:^"
   },
   "devDependencies": {
     "@backstage/cli": "workspace:^"