Split columns with different widths

### Description

Currently the `split-column` capability is great but the fact that every columns is 50% makes it for a very rigid structure. I'd appreciate if we could specify the width for each column, for example:

```
- type: split-column
max-columns: 2
widgets:
- type: rss
width: 30%
....
- type: rss
width: 70%
...
```

Alternatively, a more readable taxonomy could also be defined (but harder to implement?): `one-third, two-thirds, ...`

I have tried applying custom CSS but the output not correct, as the 50% width is fixed at an upper level

Start by locating the split-column capability and the code that fixes each column at 50% width. Determine how width values or the proposed one-third/two-thirds taxonomy should be represented in the configuration, then verify that split-column layouts honor the configured proportions.

Written by the indexing model from the issue text.