Plugin: eleventy

Implement the `eleventy` plugin for muri.

## Reference
- Knip implementation: https://github.com/webpro-nl/knip/tree/main/packages/knip/src/plugins/eleventy

## Tasks
- [ ] Create `eleventy.rs` in `crates/muri/src/plugin/`
- [ ] Implement `Plugin` trait with `name()`, `should_enable()`, `detect_entries()`
- [ ] Register in `mod.rs`
- [ ] Add tests

Parent issue: #1

Start by reading the Knip Eleventy implementation linked in the issue and the existing plugin code under crates/muri/src/plugin/. Create crates/muri/src/plugin/eleventy.rs, implement the listed Plugin methods, register it in mod.rs, and add tests. Done means the plugin is registered and its behavior is covered by tests.

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