feat(linter): allow project component sub-tokens

[Arshgill01]

Fixes #171.

Problem

Custom component sub-tokens survive parsing and reference resolution, but broken-ref warns for every occurrence. Expected extension warnings then hide real misspellings such as gaap among declared keys such as gap and owner. Disabling broken-ref is too broad because it also disables unresolved-reference errors.

Solution

• add per-rule options to the lint runner and public lint API
• let broken-ref accept additionalComponentSubTokens and union them with the built-in vocabulary
• preserve default behavior, typo warnings, and unresolved-reference errors
• export the option and descriptor types and document the programmatic API

Validation

• npx --yes bun@1.3.9 test: 348 passed, 1 optional DTCG conformance test skipped, 0 failed
• npm run lint --workspace=@google/design.md: passed
• npx --yes bun@1.3.9 run build: passed
• npx --yes bun@1.3.9 run packages/cli/scripts/check-package.ts: 25/25 passed
• packed-artifact E2E in a clean temp project: baseline warned for gap, owner, and gaap; configured lint warned only for gaap

[Arshgill01]

Status / unblock ask

mergeable_state is UNSTABLE because the Test workflow (and the Google Actions Workflow Security Scan) are stuck in action_required for this fork PR — they never ran lint/test/build.

• CLA: pass
• GitHub Actions Scan (pull_request_target): pass
• Conflicts: none (MERGEABLE, rebaseable: true)

Could a maintainer Approve and run workflows on https://github.com/google-labs-code/design.md/actions/runs/33332070373 so Test goes green? Happy to rebase onto latest main and re-request review once CI is allowed to run.

Fixes #171.