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/stacktrace_relative.test

@@ -0,0 +1,64 @@
+@args --debugger
+  nix-repl> let f = _: throw "x_x"; x = f 5; in x
+  error: x_x
+
+absolute indices still work:
+  nix-repl> :st 1
+
+  1: while calling a function
+  «string»:1:12
+
+      1| let f = _: throw "x_x"; x = f 5; in x
+        |            ^
+
+  Env level 0
+  static: _
+
+  Env level 1
+  static: f x
+
+  Env level 2
+  static:
+
+  Env level 3
+  abort baseNameOf break builtins derivation derivationStrict dirOf false fetchGit fetchMercurial fetchTarball fetchTree fromTOML import isNull map null placeholder removeAttrs scopedImport throw toString true
+
+index with + goes up the stack relative to current (1 in this case):
+  nix-repl> :st +3
+
+  4: while evaluating a 'let' expression
+  «string»:1:1
+
+      1| let f = _: throw "x_x"; x = f 5; in x
+        | ^
+
+  Env level 0
+  static: f x
+
+  Env level 1
+  static:
+
+  Env level 2
+  abort baseNameOf break builtins derivation derivationStrict dirOf false fetchGit fetchMercurial fetchTarball fetchTree fromTOML import isNull map null placeholder removeAttrs scopedImport throw toString true
+
+index with - goes down and is also relative to current (4):
+  nix-repl> :st -1
+
+  3: while calling a function
+  «string»:1:29
+
+      1| let f = _: throw "x_x"; x = f 5; in x
+        |                             ^
+
+  Env level 0
+  static: f x
+
+  Env level 1
+  static:
+
+  Env level 2
+  abort baseNameOf break builtins derivation derivationStrict dirOf false fetchGit fetchMercurial fetchTarball fetchTree fromTOML import isNull map null placeholder removeAttrs scopedImport throw toString true
+
+quit
+  nix-repl> :quit
+  error: x_x