4939: Introduce the `STARTS WITH` filter operator r=irevoire a=Kerollmops
This PR fixes#4872 by introducing the `STARTS WITH` filter operator and gating it under the _contains filter_ experimental feature along with the `CONTAINS` one. I also updated [the experimental feature discussion page](https://github.com/orgs/meilisearch/discussions/763).
Co-authored-by: Clément Renault <clement@meilisearch.com>