feat(action): add an output for the changelog content (#4)

* feat: add an output for the changelog content

::set-output is
[deprecated](https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/)
in favor of [environment
files](https://docs.github.com/en/actions/using-workflows/workflow-commands-for-github-actions#environment-files).
This has better multiline support, so add the contents of the changelog as an output for cases where
you are creating a GitHub release.

BREAKING CHANGE: self-hosted runners will need to be at 2.297.0 or greater

* fix: update multiline output usage

https://docs.github.com/en/actions/using-workflows/workflow-commands-for-github-actions#multiline-strings

* docs(readme): add content as output

Co-authored-by: Orhun Parmaksız <orhunparmaksiz@gmail.com>

action.yml

@@ -12,6 +12,8 @@ inputs:
 outputs:
   changelog:
     description: "output file"
+  content:
+    description: "content of the changelog"
 runs:
   using: "docker"
   image: "Dockerfile"