refactor: name the contract precondition clause requires

[sgraf812]

This PR spells the precondition clause of a def contract requires, pairing with ensures.

The parser declaration Lean.Parser.Command.requireClause is renamed to requiresClause accordingly.

[mathlib-lean-pr-testing]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase b1722adad3d00ad4443a08709b1efb93a78b477c --onto 0bfc3acaef4ed0576307a77fbaa0c6e1a5dca402. You can force Mathlib CI using the force-mathlib-ci label. (2026-07-29 12:26:08)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 9f011df29fa925487af67848f9b3a2b900dbb17a --onto 0bfc3acaef4ed0576307a77fbaa0c6e1a5dca402. You can force Mathlib CI using the force-mathlib-ci label. (2026-07-29 13:37:54)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase b1722adad3d00ad4443a08709b1efb93a78b477c --onto a39eab69e1eee9ad38f4efe507907b1026a77808. You can force reference manual CI using the force-manual-ci label. (2026-07-29 12:26:09)
• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 9f011df29fa925487af67848f9b3a2b900dbb17a --onto a39eab69e1eee9ad38f4efe507907b1026a77808. You can force reference manual CI using the force-manual-ci label. (2026-07-29 13:37:55)