Add new spec: TC39 legacy RegExp features

### URL

https://github.com/tc39/proposal-regexp-legacy-features

### Rationale

Via https://github.com/w3c/browser-specs/pull/2353

Unfortunately, the spec is embedded in the README. We need more flexibility in browser-specs, Reffy, and other tooling to accommodate these types of entries.

### Additional properties

```json
{}
```