Idea: ban placeholder attribute

Context: https://github.com/github/accessibility/issues/892#issuecomment-1077764961 (must be GitHub Staff to access)

There is always a better alternative to convey information typically conveyed in a placeholder.

The issue names no files, tests, or entry points; begin by reviewing the repository's existing lint rules and the linked accessibility discussion, which may require GitHub Staff access. Done should mean the linter consistently rejects placeholder attributes, with tests documenting the expected behavior.

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