fix: Fix syntax error on activate nu when PATH contains shims

[atty303]

Background and Purpose

When nushell has shims in the PATH, activate nu outputs an incorrect script.

The problem is at the beginning of the script generated by activate nu

set,Path,C:\Program Files\Microsoft VS Code\bin;C:\Program Files\PowerShell\7

This gives the following error in nushell 0.102.

Error: nu::shell::external_command

  × External command failed
   ╭─[entry #55:1:1]
 1 │ set,Path,C:\Program Files\Microsoft VS Code\bin;C:\Program Files\PowerShell\7
   · ─────────┬─────────
   ·          ╰── Command `set,Path,C:\Program` not found
   ╰────
  help: `set,Path,C:\Program` is neither a Nushell built-in or a known external command

This is not a Nu script, but a CSV protocol.
The nushell functions defined in activate (def “parse vars”) parse CSV only for deactivate, shell and sh command output.
The CSV protocol is not available in activate itself, so it must output valid Nu scripts.

Changes

• Shell::set_env should change the output if it is running in the context of activate. I introduced enum ActivatePrelude to avoid making it mutable.

Impact

• The output of other shells is kept the same.