feat(eslint-plugin): add no-closure-in-durable-operations rule

[ParidelPooya]

• Add new rule to detect and prevent closure variable mutations in durable operations (step, runInChildContext, waitForCondition, waitForCallback)
• Rule allows reading closure variables but prevents assignments and updates (=, +=, ++, --)
• Recursively tracks variable declarations across nested scopes
• Reorganize rules into subdirectories with rule implementation and tests co-located
• Add unit tests covering mutations, reads, and scope handling
• Update README with new rule documentation and examples

Limitations:

1. Object/Array Mutations: The rule only catches direct assignments and update expressions but misses property mutations:

let obj = { count: 0 };
await context.step(async () => {
  obj.count++; // ❌ Not detected but equally problematic
  obj.items.push(1); // ❌ Not detected
});

2. Destructuring Assignments:

let a = 1, b = 2;
await context.step(async () => {
  [a, b] = [b, a]; // ❌ Not detected
});

3. Performance Consideration: The walkNode function recursively traverses the entire callback AST. For large callbacks, this could be slow. I'll investigate ESLint's built-in visitor pattern instead of manual traversal later.

4. Duplicate Error in Nested Cases: Nested operations report the error twice (once for each level). This might be confusing to users - I'll enhance it later to report at the innermost level.

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