…ps upload (#14942)

- replace the original release management plugin with a custom one
where we start release creation on `closeBundle`
- replace the original file deletion plugin with a custom one where we
start deletion on `closeBundle`
- in both cases, further ensuresthat the plugin is only invoked for
build and as late as possible (`enforce: 'post'`). 
- searche for the release management plugin by its old and new name
introduced in
getsentry/sentry-javascript-bundler-plugins#647
- slightly rename the custom source maps uploading plugin for better
convention; as well as some variables for better readability
- add unit tests for the new custom sub plugins