add header_on_separator options to default_config.nu (#9922)

# Description

This PR adds the `header_on_separator` table option as `false` to the
`default_config.nu` file.

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

@@ -164,6 +164,7 @@ $env.config = {
             wrapping_try_keep_words: true # A strategy used by the 'wrapping' methodology
             truncating_suffix: "..." # A suffix used by the 'truncating' methodology
         }
+        header_on_separator: false # show header text on separator/border line
     }
 
     # datetime_format determines what a datetime rendered in the shell would look like.