Rename step: to func: for GitLab Functions compatibility

Overview

Update the Step Runner to support the new func: reference syntax as part of the GitLab Functions naming initiative. This maintains backwards compatibility while aligning with the rebranded terminology.

Problem Statement

GitLab CI Steps has been renamed to GitLab Functions. The Step Runner currently only recognizes the step: syntax, which creates friction for users adopting the new naming convention.

Solution

Add support for the func: reference syntax while maintaining full backwards compatibility with existing step: references.

Acceptance Criteria

• func: syntax is recognized and resolves functions correctly
• step: syntax continues to work without any changes
• Both syntaxes can be used interchangeably in the same configuration
• Documentation is updated to reflect both supported syntaxes
• Existing tests pass; new tests cover both syntax variants