Expose `strategy` and `matrix` variables in `context`

Start with actions/github-script@v7 and trace how the script receives its context object, then compare the available GitHub Actions strategy and matrix values with the requested names. Done means workflow scripts can access the requested values through context, with tests or examples covering matrix and strategy usage.

Is your feature request related to a problem? Please describe.

I'm using a matrix and I want to access matrix- and strategy-related variables in my github-script. The two ways to do that are:

1. Setting env vars:

# ...
strategy:
  matrix:
    group: [1, 2]
steps:
  - uses: actions/github-script@v7
    env:
      MATRIX_GROUP: ${{ matrix.group }}
      MATRIX_NUM_GROUPS: ${{ strategy.job-total }}
    with:
      script: |
        const group = parseInt(process.env.MATRIX_GROUP)
        const numGroups = parseInt(process.env.MATRIX_NUM_GROUPS)

2. Interpolation:

# ...
strategy:
  matrix:
    group: [1, 2]
steps:
  - uses: actions/github-script@v7
    with:
      script: |
        const group = ${{ matrix.group }}
        const numGroups = ${{ strategy.job-total }}

Describe the solution you'd like
It would be nice if these were offered somewhere in the context object, which seems like an intuitive home for them. Something like...

# ...
strategy:
  matrix:
    group: [1, 2]
steps:
  - uses: actions/github-script@v7
    with:
      script: |
        const group = parseInt(context.strategy.matrix.group)
        const numGroups = context.strategy.totalJobs