Allow non-percent values to be used for column-width

[aristath]

When building a FSE template, themes, designers & authors will need a lot more than just % values.
The simplest and most common scenario: A header with a 200px-wide column on the left for the logo while the menu is on the right and takes up the remaining available space. Percent values won't do 'cause we don't want the logo to shrink.
Ideally we'd have something like #18600, but since that idea never took off, we're forced to try and build things using nothing but group and column blocks.
We'll need something more versatile than hardcoded % values.

A possible solution would be to add a "custom" switch next to the current width controls we have in columns. If selected, it would show a textfield where users can enter what they need.
A numeric field with a units-selector next to it like it's being discussed for some global styles will do for most cases but probably won't cover things like calc(100% - 200px).

[aristath]

Fixed in #24711