This commit refactors the existing tools, by moving the tools logic to a
new `tools.ts` file, so that the main `cmd.ts` file remains clear.
Besides that we also add a new tool `get-feed` which can be used to run
the `getFeed` function from the command line. The function is called
with a source and returns the generated source and items, as they are
saved in the database by the `add-source-v1` Supabase edge function.