check Nushell files in the CI

in order to help catch issues with the various Nushell scripts and modules in the source base, i propose to add a CI script that will check them all.

below is an example error, e.g. when introducing a syntax error in .env.nu:

Error:   × Failed to parse content:
  │     file: .env.nu
  │     err:  Expected keyword.

scripts/check-nushell-files.nu

@@ -13,8 +13,8 @@ for f in $files {
 
         error make --unspanned {
             msg: ([
-                $"($e.msg):",
-                $"    file: ($f)",
+                $"(ansi red_bold)($e.msg)(ansi reset):",
+                $"    file: (ansi purple)($f)(ansi reset)",
                 $"    err:  ($err.msg)",
             ] | str join "\n")
         }