fix: use referenced names in descriptions

[pupuking723]

Fixes #1064.

This keeps the aria-describedby referenced subtree in referenced context while computing text alternatives, so descendants with aria-label contribute their accessible name to the computed description instead of falling back to raw text content.

It also updates the existing WPT expectations for description_from_content_of_describedby_element-manual.html from fail to pass.

Verification:

• yarn test sources/tests/accessible-description.js --runTestsByPath --runInBand
• yarn test
• yarn prettier --check sources/accessible-name-and-description.ts sources/tests/accessible-description.js tests/wpt-jsdom/to-run.yaml tests/cypress/integration/web-platform-test.cy.js .changeset/describedby-referenced-names.md
• yarn lint
• yarn test:types
• yarn build

Note: yarn test:wpt:jsdom could not complete locally because initializing the tests/wpt submodule stalled while cloning the upstream WPT repository.