Make matching_strategy a literal with all, last, and frequency

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (7cc470e) to head (f86c17b).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## meilisearch-1.9.0 #954 +/- ## =================================================== Coverage 100.00% 100.00% =================================================== Files 19 19 Lines 2839 2839 =================================================== Hits 2839 2839 ```

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