Adds platform agnostic EoL separator to char command (#15059)

Adds `char eol`, `char line_sep`, and `char lsep` (synonyms) to represent
the platform specific line ending character(s).

crates/nu-command/tests/commands/platform/char_.rs

@@ -8,5 +8,5 @@ fn test_char_list_outputs_table() {
         "#
     ));
 
-    assert_eq!(actual.out, "107");
+    assert_eq!(actual.out, "113");
 }

crates/nu-command/tests/commands/platform/mod.rs

@@ -1,2 +1,3 @@
 mod ansi_;
+mod char_;
 mod kill;