Rename root module to `Meilisearch` instead of `MeiliSearch`

[ellnix]

Also, if you have time you could put in your big breaking changing the main class name to Meilisearch instead of MeiliSearch. Not only here but in the ruby gem as well.

Originally posted by @brunoocasali in https://github.com/meilisearch/meilisearch-rails/pull/344#discussion_r1507561897

Also attempt to maintain backwards compatibility, with a warning for usage of MeiliSearch.