remove Nushell

because of many problems with Nushell ™️, this removes everything Nushell-related

changelog

• all mentions to Nushell have been removed
• all CLI implementations and tests written in Nushell have been removed
• the Makefile has been updated
• the test assets have been moved one directory up
• extra
  • rust-analyzer has been added to the toolchain
  • pin Tokio to 1.43.0 to avoid unpredictable CI breakage

related work

this should allow deleting the following branches

• work-on-tests
• fix-nushell-syntax

this should supersede the following

• !25 (closed)