Rebuild Growler into web component

[tvdeyen]

What is this pull request for?

Adds a new <alchemy-message> web component that is used by Alchemy.growl, render_flash_message and render_message helpers.

Checklist

• [x] I have followed Pull Request guidelines
• [x] I have added a detailed description into each commit message
• [x] I have added tests to cover this change

[codecov[bot]]

Codecov Report

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

Project coverage is 95.88%. Comparing base (69df895) to head (4261254).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2795 +/- ## ========================================== + Coverage 95.81% 95.88% +0.07% ========================================== Files 222 223 +1 Lines 6087 6082 -5 ========================================== Hits 5832 5832 + Misses 255 250 -5 ```

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