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.

• 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 formatting and comments.

• Codegen module:

  • index.ts orchestrates
  • builders.ts constructs new AST nodes from selected presets and options
  • merge-target.ts reads and updates the user's AST
  • print.ts runs recast.print and an optional Prettier to polish the formatting