`FontSize` implements `Component` but does not function when used as a component

## Bevy version and features

version = 0.19.0
features = ["2d", "audio", "debug", "ui"]

## What went wrong

`FontSize` is a `Component`, but it doesn't work when attached to an entity. Instead, it only works when it is set on the `font_size` field of `TextFont`.

I think the easiest solution is to remove `Component` from the derives of `FontSize`, but I'm not sure whether `TextFont` is being split up into components.

Locate the definitions and derives for FontSize and TextFont, then reproduce the issue by attaching FontSize to an entity rather than setting TextFont.font_size. Check the surrounding text component design and existing tests to determine whether FontSize should be removable as a Component or independently functional; done means the intended usage is consistent and covered by a regression test.

Written by the indexing model from the issue text.