refactor "inbreeding" Nushell modules

this MR

• moves the Nushell modules from bins/inbreeding/ to bins/inbreeding/src/.nushell/
• creates a NUSHELL constant in consts.nu to allow the following more robust construct

use consts.nu
use $consts.NUSHELL ...

• updates the README

bins/inbreeding/src/.nushell/consts.nu

 export const BIN = "./target/release/inbreeding"
 export const CACHE = ($nu.home-path | path join .cache komodo inbreeding)
+
+export const NUSHELL = "../../../../.nushell"