add custom children support

[bugzpodder]

This allows custom children to be defined instead of fixed hardcoded layouts (ie both/both_flipped)

https://codesandbox.io/p/sandbox/bold-napier-ytln8u

[jjti]

I merged and released it in a new minor version @bugzpodder. Please lmk if all looks well when you try it out

Also @bugzpodder this is the biggest contribution by anyone outside of Lattice. Would you like attribution in a Contributor section in the README or something to that effect? Thanks again -- for this and the ongoing work on the single-row linear viewer. This is awesome stuff

[bugzpodder]

i'll pull the new minor into my app next week and try to use the new feature. will let you know if everything looks good or not. happy to continue to contribute back either way, feel free to add my tag once you have a contribution section, leaving it up to you, thanks!