Add max-parallel support for matrix strategy

[beagleknight]

Summary

• Adds an optional maxParallel property to the Matrix interface, which compiles to max-parallel under the strategy block in the generated GitHub Actions workflow YAML.
• When maxParallel is not set, the output is unchanged (no max-parallel key emitted).
• Includes a new test case verifying the feature with updated snapshots.

Usage

workflow.addJob("job1", {
  matrix: {
    elements: [
      { id: "node", options: [18, 20, 22] },
    ],
    maxParallel: 2,
  },
  steps: [{ run: "echo ${{ matrix.node }}" }],
});

Produces:

strategy:
  fail-fast: false
  max-parallel: 2
  matrix:
    node:
      - 18
      - 20
      - 22