feat(no-useless-fallback-in-spread): add no-useless-fallback-in-spread rule

[bartlomieju]

This ports the unicorn no-useless-fallback-in-spread rule from oxlint as a
native deno_lint rule.

The rule flags a useless empty-object fallback inside an object spread, such as
{ ...(foo || {}) } or { ...(foo ?? {}) }. Spreading a nullish (or otherwise
falsy) value into an object literal is a no-op, so providing an empty object
{} as a fallback via || or ?? adds nothing and can be removed. The rule
only triggers when the spread is inside an object literal (not an array literal
or call arguments), the logical operator is || or ??, and the right operand
is an empty object literal, unwrapping any surrounding parentheses to match
oxc's behavior.

Reference implementation:
https://github.com/oxc-project/oxc/blob/main/crates/oxc_linter/src/rules/unicorn/no_useless_fallback_in_spread.rs

Tagged RECOMMENDED (on by default) — flagging for maintainer sign-off.

[bartlomieju]

Verdict: LGTM — correct and well-tested. Recommend merge after the RECOMMENDED sign-off you flagged + rebase. Not auto-merging (new rule).

Clean port; flags { ...(foo || {}) } / { ...(foo ?? {}) }.

Correctness — verified

• Scoping is elegant: hooking object_lit means only object-literal spreads are checked, so array spreads ([...(foo || {})]), call-arg spreads (call({}, ...(foo || {}))), and destructuring patterns (const {...foo} = obj) are naturally excluded — all covered by valid tests.
• Operator gate is correct: only LogicalOr/NullishCoalescing, so {...(foo && {})} isn't flagged (matches oxc), and only the right operand is checked, so {...({} || foo)} isn't flagged.
• Empty-object check via object.props.is_empty() correctly treats {...(foo || {...{}})} (right object has a spread prop) and {...(foo || {not: "empty"})} as non-empty.
• unwrap_parens is applied to both the spread expr and the right operand, and non-Bin spread exprs ({...(foo || {}).toString()}, {...fn(foo || {})}, {...(foo ? foo : {})}) are correctly skipped.

Perf: trivial — per object_lit, linear over its props.

Housekeeping: RECOMMENDED sign-off (you flagged it); mergeable: UNKNOWN → rebase; docs .md not in diff.

No code changes requested — the object_lit-scoped approach cleanly handles every edge in the reference. Just tagging + rebase.