Abstract build steps to externalize the build configuration

[alfonso-noriega]

[Feature] Add build_steps extension build mode for flexible build pipelines

WHY are these changes introduced?

This PR introduces a new build_steps build mode for extensions, providing a more flexible and configurable build pipeline system. This allows extension developers to define custom build steps with different strategies, rather than being limited to the existing predefined build modes.

WHAT is this pull request doing?

• Adds a new build_steps mode to the extension build configuration
• Implements a build steps pipeline executor that can run steps sequentially or in parallel
• Creates a copy_files step implementation with three strategies:
  • directory: Copy an entire directory
  • pattern: Copy files matching glob patterns
  • files: Copy a specific list of files
• Adds comprehensive test coverage for the new build steps system
• Integrates the new build mode with the existing extension build process

The architecture follows a Command Pattern approach, where each step type has a dedicated executor, making it easy to add more step types in the future.

How to test your changes?

1. TODO
2. Run shopify app dev and verify that the assets are correctly copied to the output directory
3. Try different copy strategies and configurations to test the flexibility of the system

Measuring impact

How do we know this change was effective? Please choose one:

• n/a - this doesn't need measurement, e.g. a linting rule or a bug-fix

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes

[alfonso-noriega]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• Abstract build steps to externalize the build configuration #6842 👈 (View in Graphite)
• Asset pipeline flow for hosted static app #6806
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.

[github-actions]

Coverage report

St.❔	Category	Percentage	Covered / Total
🟡	Statements	79.74%	14604/18315
🟡	Branches	74.04%	7257/9802
🟡	Functions	79.88%	3708/4642
🟢	Lines	80.09%	13809/17242

Test suite run success

3777 tests passing in 1461 suites.

Report generated by 🧪jest coverage report action from cf3d1cc