Do not pass non-string env vars to externals (#4748)

* Do not pass non-string env vars to externals

Also misc cleanup

* Add note to default config

* Add a test

* Ensure PATH/Path conversion list <-> string

docs/sample_config/default_config.nu

@@ -28,6 +28,7 @@ let-env PROMPT_MULTILINE_INDICATOR = "::: "
 # Specifies how environment variables are:
 # - converted from a string to a value on Nushell startup (from_string)
 # - converted from a value back to a string when running extrnal commands (to_string)
+# Note: The conversions happen *after* config.nu is loaded
 let-env ENV_CONVERSIONS = {
   "PATH": {
     from_string: { |s| $s | split row (char esep) }