Add new `version-file-exists` input parameter

• Add new (optional, default is true) input parameter version-file-exists:
  Then version-file-exists: true specified by version-file file must indeed exist in repository. Otherwise the action fails with [version-file] could not be found but is required error
• When version-file-exists: false and there is no version-file in repository the action now only sets VERSION environment variable and project-version action output to calculated project version