Add support for the extends keyword

[Sam Frost (SamuelFrost)]

(note for reviewers: this is vibe coded based off the existing PR and issues. I have not reviewed it yet beyond minimal local testing-- hence the draft status)

Summary

Implements configuration inheritance from spec#22: a devcontainer.json can inherit another JSON or JSONC file in the same repository via extends.

This reapplies PR #311 on current main. That branch could not be rebased as a delta. The merge path follows the review on #311: use the image metadata merge logic, not deepmerge.

Companion spec PR: devcontainers/spec#772

Does not bump the package version. That belongs in a later release PR.

Changes

• Add optional extends?: string on the devcontainer.json config types.
• Resolve extends when reading configuration: relative paths only, recursive, with cycle detection. Absolute paths and URLs are rejected.
• Merge the referenced file with the current file using the existing image metadata rules (forwardPorts / capAdd union, hostRequirements max, last-wins for scalars, init / privileged true if any value is true). Strip extends from the result.
• Add tests for a single reference, nested references, cycles, a missing file, and overlapping merge properties.
• Note the change under Unreleased in CHANGELOG.md without cutting a version.

Technical choices

#311 used deepmerge. Christof Marti, Josh Spicer, and Chuck Lantz asked to reuse image metadata merge so extending a file matches combining a prebuilt image’s metadata with a project devcontainer.json. mergeDevContainerConfigs in imageMetadata.ts is that shared path.

Relative-path-only matches the spec issue (same repository). Recursive extends is included with cycle detection; #311 left nested references unresolved.

Validation

• src/test/configContainer.test.ts: 6 passing (read a basic file, resolve extends, nested extends, cyclic reference, missing file, merge overlap).
• yarn type-check
• yarn lint

devcontainer up was not run as part of this change. read-configuration against a file that extends another produces the merged config and omits extends.

Follow-up

After maintainers are on board, CONTRIBUTING.md asks for a docs PR to the devcontainer.json reference in vscode-docs.

Implements spec#22. Supersedes #311.

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