[2.x] Rename method `Asset::hasMediaFile` to `Asset::exists`

[caendesilva]

Also considered has, but exists is generally better for clarity, as it directly indicates that you're checking for the presence of a file. has can be more ambiguous, as it might imply ownership or possession rather than existence. Therefore, exists is often preferred for methods that check for the presence of an item. It's also what the Laravel Filesystem facade uses.

Part of https://github.com/hydephp/develop/pull/1904

[codecov[bot]]

Codecov Report

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

Project coverage is 100.00%. Comparing base (8dfb5ce) to head (a897d0d). Report is 2 commits behind head on normalize-the-asset-api.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## normalize-the-asset-api #1957 +/- ## =========================================================== Coverage 100.00% 100.00% Complexity 1883 1883 =========================================================== Files 193 193 Lines 4977 4977 =========================================================== Hits 4977 4977 ```

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