refactor(preset-basic): move helper functions to @effect-migrate/core

[aridyckovsky]

Summary

Move getLineColumn and findMatchingBrace helper functions from @effect-migrate/preset-basic to @effect-migrate/core utilities for reuse across custom presets.

Motivation

Currently, these helpers are defined in packages/preset-basic/src/helpers.ts:

• getLineColumn(content, index) - Converts character index to line/column numbers
• findMatchingBrace(content, startIndex) - Finds matching closing brace

Custom presets and rule authors would benefit from having these utilities available from core, avoiding code duplication.

Proposed Solution

1. Create packages/core/src/utils/helpers.ts
2. Move getLineColumn and findMatchingBrace to core
3. Export from packages/core/src/index.ts
4. Update @effect-migrate/preset-basic to import from core
5. Add unit tests in packages/core/test/utils/helpers.test.ts

Benefits

• Promotes DRY (Don't Repeat Yourself)
• Makes custom preset development easier
• Ensures consistent helper implementations across presets
• Reduces bundle size for packages that depend on multiple presets

Files to Change

• packages/core/src/utils/helpers.ts (new)
• packages/core/src/index.ts (export helpers)
• packages/core/test/utils/helpers.test.ts (new)
• packages/preset-basic/src/helpers.ts (remove or re-export from core)
• packages/preset-basic/src/patterns.ts (update imports)

Acceptance Criteria

• Helpers exported from @effect-migrate/core
• preset-basic imports helpers from core
• All existing tests pass
• New tests added for helpers in core
• Documentation updated

Related

• Identified during PR review: feat(cli,preset-basic): implement first-class preset support
• Thread: https://ampcode.com/threads/T-09db5199-2c22-42fd-9730-3a6f1c42e570