Clear definitions cache after file change

[tvdeyen]

What is this pull request for?

When the elements.yml or page_layouts.yml files change the memoiziation gets resetted, so that the next app reload includes the latest changes to these files.

This removes the need to restart the Rails server after one changed an element definition or page layout.

Checklist

• [x] I have followed Pull Request guidelines
• [x] I have added a detailed description into each commit message

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 73.68421% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 95.86%. Comparing base (0c76560) to head (0105c6e).

Files	Patch %	Lines
lib/alchemy/engine.rb	54.54%	5 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2824 +/- ## ========================================== - Coverage 95.92% 95.86% -0.06% ========================================== Files 225 225 Lines 6128 6143 +15 ========================================== + Hits 5878 5889 +11 - Misses 250 254 +4 ```

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