Add linting support for cache-mode

**Is your feature request related to a problem? Please describe.**
With the release of [cache-mode](https://github.blog/changelog/2026-09-10-control-github-actions-cache-access-with-cache-mode/), it's time to add linting support for the property!

**Describe the solution you'd like**
Currently, the extension displays an error for `cache-mode`.
The solution would be to add linting support for `cache-mode` like other properties, e.g. concurrency and jobs.

**Additional context**
Changelog post: https://github.blog/changelog/2026-09-10-control-github-actions-cache-access-with-cache-mode/

Start by locating the linting definitions for existing properties such as concurrency and jobs, then inspect how the VS Code extension reports the currently invalid cache-mode property. Done means cache-mode is accepted and linted like the other supported properties without producing an error.

Written by the indexing model from the issue text.