feat(create-eslint-config): extend existing config

[hanna-skryl]

Closes #47

The wizard now extends an existing eslint.config.js instead of printing a manual-merge snippet. Adds AST-based codegen, atomic flush rollback, and ESM/CJS detection precedence.

• ESM-first detection. When both eslint.config.mjs and eslint.config.cjs exist in a project, the wizard picks the ESM file.

• All-or-nothing file writes. tree.flush() writes files sequentially. If any write fails, originals are restored and freshly-created files are deleted before the error rethrows, so the project is never left in a half-written state.

• Recast-based codegen. The wizard parses the user's source with recast, mutates the AST, and reprints. Untouched nodes keep their original bytes, so user comments and formatting outside the inserted regions remain intact.

• Codegen module:

  • index.ts orchestrates
  • builders.ts constructs new AST nodes from selected presets and option blocks
  • merge-target.ts reads, recognizes, and mutates the user's AST
  • print.ts runs recast.print and an optional Prettier finalize.