libcmd/repl: allow :st argument to be relative to current stack index

See [lix-project/lix#1156], but basically currently the `:st <n>`
debugger command doesn't allow any negative indices, and putting a plus
sign in front of the arg doesn't change anything; thus, we can exploit
that "design space" to allow users to move between different stack
frames easily, by simply prepending their arg with a +/- sign.

The actual behavior is little more nuanced when you account for errors:
as suggested by @pennae (thanks! :), when the user inputs an offset that
would result in an invalid frame index, the debugger instead clamps it
to the closest bound (i.e. 0 for negative offsets, $maxFrame for
positive ones) and just prints a warning.

Fixes #1156

[lix-project/lix#1156]: https://git.lix.systems/lix-project/lix/issues/1156

Change-Id: I02a0cdb6aaebbdb0515308880a3bf9c0d2fcd25e

tests/functional/repl_characterization/data/debug_frames.test

@@ -40,7 +40,7 @@
     :bt, :backtrace                       Show trace stack
     :c, :continue                         Go until end of program, exception or builtins.break
     :s, :step                             Go one step
-    :st, :show-trace [integer index]      Show current trace. If an integer is provided, this switches to that stack beforehand.
+    :st, :show-trace [integer index]      Show current trace. If an integer is provided, this switches to that stack beforehand. If the integer has an explicit + or - sign, it is treated as relative to the current stack index.
 
       Flakes commands