[6.x] New folder ux improvements

[JayGeorge]

This PR should fix #12457, improving the "create folder" interactions.

It's not quite perfect, but a lot better than the current UX.

1. When you create a new folder, the placeholder text is lighter. It's gray-400 rather than gray-500. While technically this is not quite as color-contrast accessible, on balance it feels more obvious that it's placeholder text now.

2. If you just hit enter the text turns red, prompting you to fix the name (in addition to the toast that was already there).

3. If you enter a valid value and hit enter, the folder will be created.

So this is where it gets a little gnarly…

Since a Reka component is used here to manage the [data-placeholder-shown], it becomes tricky to target the error state. There may be a better way, but I figured out I could add an error class to the hidden input, then target that with CSS.

The only downside is that when entering a correct value, it will temporarily appear red before being created—I guess while the validation is happening and the error is being removed.