Ensure default config files end with a new line (#14192)

# Description

This allows users to immediately add new lines to the configuration in
the usual way.

crates/nu-utils/src/sample_config/default_config.nu

@@ -896,4 +896,4 @@ $env.config = {
             event: { edit: selectall }
         }
     ]
-}
+}

crates/nu-utils/src/sample_config/default_env.nu

@@ -98,4 +98,4 @@ $env.NU_PLUGIN_DIRS = [
 # $env.PATH = ($env.PATH | uniq)
 
 # To load from a custom file you can use:
-# source ($nu.default-config-dir | path join 'custom.nu')
+# source ($nu.default-config-dir | path join 'custom.nu')

crates/nu-utils/src/sample_config/sample_login.nu

@@ -6,4 +6,4 @@
 $env.PROMPT_INDICATOR = {|| "(LS)> " }
 
 # Similar to env-path and config-path there is a variable containing the path to login.nu
-echo $nu.loginshell-path
+echo $nu.loginshell-path