FR: Extract code to new val

[nbbaier]

It would be nice to have a command that could take selected code in a file/val and extract it to a new file. The workflow I imagine is like this:

1. Select some text
2. Trigger "Extract to new val" (or similar) command
3. Extension creates new val from that text

It would also perhaps be nice to have an editing step in there before "committing" the new Val to the Val Town.

Also, as @neverstew has suggested on the discord, it might be possible to use VSCode's built in Refactor command to do this to some extent.