Implement feature installation order and enhance test suite

[prulloac]

This pull request adds support for the installsAfter (soft dependency) and dependsOn (hard dependency) fields to the devcontainer features specification, and introduces comprehensive internal tests to validate feature installation order logic, including handling of dependency resolution, override semantics, and cycle detection. Documentation has also been updated to reflect the new support status for these fields.

Feature dependency enhancements:

• Added installsAfter (soft ordering hint) and dependsOn (hard dependency) fields to the Spec struct in features.go, enabling features to declare installation order preferences and requirements.

Testing and validation:

• Introduced a new internal test suite in install_order_internal_test.go to thoroughly test the dependency and installation order resolution logic, covering cases such as ambiguous references, override precedence, cycle detection, and complex dependency trees.

Documentation updates:

• Updated docs/devcontainer-spec-support.md to indicate support for the overrideFeatureInstallOrder, dependsOn, and installsAfter fields, replacing the previous "unsupported" status. [1] [2]