Refactor: extract to a new val

[neverstew]

I often write things in roughly this order (in Val Town or otherwise:

1. Dump everything in one place.
2. Extract complex sections to named functions
3. Tidy and make interfaces more consistent

Steps 2 and 3 are currently quite difficult on Val Town. I think having an automated way to extract a named function to a separate Val would do wonders for enabling composability (with other people as well!).

[nbbaier]

I'd really like this. I wrote a very minimal extension to do it at one point and might do it again

[neverstew]

I found this a helpful starting point in understanding what would need to happen for this to work well:

https://stackoverflow.com/questions/69673743/applying-a-typescript-refactoring-programmatically