style: Move `float` CSS property outside mako
Hi, I am fairly new to Rust and this is my first contribution to a Rust project.
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix#21130
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- [ ] There are tests for these changes OR
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I'm not sure if the approach with the two enums is good. Please let me know if this can be improved.
Regarding the tests I'm unsure: `tests/wpt/web-platform-tests/css/css-display/display-contents-float-001.html` seems to fail already before this PR, (and the cause is maybe not related to the style property definition).
Also I would like to add some docs, is it allowed to use the descriptions from MDN (https://developer.mozilla.org/en-US/docs/Web/CSS/float).
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