debugger: swap cmd_up and cmd_down commands

Historically luadebug.lua used `up` and `down` in
a strange order which was inherited from original
debugger.lua implementation. But that was counter
intuitive, and is confusing.

Swap their meaning to be more compatible with
that we accustomed in `gdb`/`lldb`.

NO_DOC=internal

changelogs/unreleased/gh-7790-cmd-up-down-swapped.md

+## feature/debugger
+
+* Swapped 'up' and 'down' commands in debugger - to make them behave more
+  like in `gdb`/`lldb`.

test/app-luatest/console_debugger_session_test.lua

@@ -81,10 +81,10 @@ local sequence = {
     { ['p ymd'] = 'ymd => false' },
     { ['w'] = 'local hms = false' },
     { ['h'] = dbg_prompt },
-    { ['t'] = 'debug-target.lua:5 in chunk at' },
-    { ['u'] = 'Already at the top of the stack.' },
-    { ['d'] = 'debug-target.lua:5 in chunk at' },
-    { ['u'] = 'Inspecting frame: builtin/datetime.lua' },
+    { ['t'] = '=> builtin/datetime.lua' },
+    { ['u'] = 'debug-target.lua:5 in chunk at' },
+    { ['u'] = 'Already at the bottom of the stack.' },
+    { ['d'] = 'Inspecting frame: builtin/datetime.lua' },
     { ['l'] = 'obj => {"tzoffset" = "+0300", "hour" = 3}' },
     { ['f'] = dbg_prompt },
     { ['n'] = dbg_prompt },

third_party/lua/luadebug.lua

@@ -419,7 +419,10 @@ local function cmd_eval(code)
     return false
 end
 
-local function cmd_down()
+--[[
+    Go up the stack. It advances from the callee frame to the caller.
+]]
+local function cmd_up()
     local offset = stack_inspect_offset
     local info
 
@@ -441,7 +444,10 @@ local function cmd_down()
     return false
 end
 
-local function cmd_up()
+--[[
+    Go down the stack. It advances from the caller frame to the callee.
+]]
+local function cmd_down()
     local offset = stack_inspect_offset
     local info