fix: nushell

[Ev357]

I was trying to make lmod work with nushell but i faced some issues i was able to fix:

• The environment variables are incorrectly parsed with an ' at the end that breaks the base64 encoding and bunch of other stuff i believe. I managed to fix it by correctly setting the substring indexes.
• In my case i need multiple MODULEPATH variables and they are set to an nushell list which cannot be parsed back by lmod because lua cannot read nushell list variables. I fixed this by adding MODULEPATH to ignore checks.
• When parsing tcl modulefiles it says that nushell is not a known shell. I fixed it by just including the nushell shell type.