cli: initial cli-plugin-api

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

.changeset/cli-use-cli-plugin-api.md

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+---
+'@backstage/cli': patch
+---
+
+Migrated CLI plugin modules to import from the new `@backstage/cli-plugin-api` package.

.changeset/create-cli-plugin-api.md

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+---
+'@backstage/cli-plugin-api': minor
+---
+
+Added a new `@backstage/cli-plugin-api` package that provides the public API for building Backstage CLI plugins. This includes `createCliPlugin`, `lazy`, `ExitCodeError`, `exitWithError`, and associated types.