Add a lint for combined equality assertions

[aryansk]

Fixes #1810

changelog: [assert_multiple]: Add a lint for assertions with multiple equality comparisons

Summary

• add the nursery assert_multiple lint for assert! and debug_assert! calls combining two equality comparisons with &&
• suggest separate assert_eq!/assert_ne! calls when all operands implement Debug
• skip custom assertion messages and non-Debug operands to avoid changing output or producing invalid suggestions
• add UI coverage and register the lint in the generated lint list

Validation

• cargo fmt --all -- --check
• git diff --check
• Clippy UI compilation was not available locally because this checkout lacks the rustc-dev compiler components; upstream CI is required for compiler and UI validation.

Prepared with AI assistance; I reviewed the implementation and validation output.