feat: add file-exists action

[FidelusAleksander]

Changes

Checklist

• I have added or updated appropriate labels to this PR
• I have tested my changes
• I have updated the documentation if needed

[Copilot]

Pull Request Overview

This PR introduces a new composite GitHub action to check for the existence of a file in the repository.

• Adds a new composite action defined in actions/file-exists/action.yml.
• Provides a workflow (.github/workflows/_test-file-exists-action.yml) to test both success and failure scenarios for the action.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
actions/file-exists/action.yml	Adds a composite action to verify if a file exists in the repository.
.github/workflows/_test-file-exists-action.yml	Adds tests to confirm the proper behavior of the new action.

Comments suppressed due to low confidence (1)

actions/file-exists/action.yml:18

• The 'core' module is used without being imported or defined. Consider importing '@actions/core' or using an alternative method such as throwing an error to properly fail the action.

core.setFailed(`❌ File does not exist: ${filePath}`);

[chriswblake]

One more comment: I would add a very basic README providing usage and an example. Also list it in your main readme.

[FidelusAleksander]

Note to self: update release-drafter configuration with a new section for actions + labeler with a new label