feat: add prefer-promise-all lint rule

[danielchen0]

Summary

• Adds prefer-promise-all rule that detects sequential await in for...of loops that could be parallelized with Promise.all(items.map(async ...))
• Uses scope analysis to detect cross-iteration variable dependencies (variables declared outside the loop that are both read and written inside)
• Skips order-dependent patterns: .push(), .unshift(), .splice() to arrays
• Skips loops with break/return (early exit depends on sequential evaluation)
• Skips for await...of (async iterator pattern)
• Skips await inside nested functions (not actually sequential in the loop)
• Platform: universal (no platform tag — useful on web + backend)

Heuristics

The rule flags when all of these are true:

1. Loop is for...of (not for, while, or for await...of)
2. Loop body contains a direct await (not inside a nested function)
3. No .push() / .unshift() / .splice() calls (order-dependent accumulation)
4. No break or return targeting the loop (sequential early-exit)
5. No variable declared outside the loop that is reassigned inside it (cross-iteration dependency)

Test plan

Scenario	Expected
for (const x of items) { await doThing(x); }	Warning: use Promise.all
Result added to Set/Map	Warning: use Promise.all
Result .push()ed to array	No warning (order matters)
Loop has break on condition	No warning (early exit)
Loop has return on condition	No warning (early exit)
Cross-iteration variable (let prev = ...; prev = await ...)	No warning (dependency)
Accumulator (total += await ...)	No warning (dependency)
for await...of	No warning (async iterator)
No await in loop	No warning
await inside nested .map() callback	No warning (not sequential in loop)
break inside switch (not targeting loop)	Still warns (break doesn't exit loop)