`format { frameCol }` impossible

As a follow-up for https://github.com/Kotlin/dataframe/issues/1356 and https://github.com/Kotlin/dataframe/issues/982,
we need a way to format frame columns easily.

Currently, the only way to format a `FrameColumn` is to make each `DataFrame` inside a `FormattedFrame`.
This need to happen first, though, as there's no way to call `.convert` on a `FormattedFrame`. Then the whole thing needs to become a `FormattedFrame` and you can display it as HTML:
```kt
df.convert { myFrameCol }.with {
it.format { someCol }.with { background(green) }
}
.format().with { null } // or .let(::FormattedFrame)
```

That's not just ugly, but also very hard to find and understand.

Currently, this does nothing:
```kt
df.format { myFrameCol }.with { background(green) }
```
so it might make sense turn that call into something that applies to all dataframes inside the framecolumn (that said, there should be a way to format specific columns inside those frame columns as well, but we have no notation as of yet to select or target any of those in DataFrame).