Put misc/gdb.py [experimental]

This works like:

```
(gdb) cfp
CFP (count=3, addr=0x7ffff73fef50):
$1 = {pc = 0x555556bf7818, sp = 0x7ffff72ff078, iseq = 0x7ffff2603270, self = 140737344619296, ep = 0x7ffff72ff058, block_code = 0x0,
  __bp__ = 0x7ffff72ff060, jit_return = 0x555558c2b000}

Stack (size=3):
[0] FIXNUM: 1
[1] T_STRING: "" bytesize:0 (embed) encoding:1 coderange:7bit $2 = (struct RString *) 0x7ffff249ea80
[2] [PROMOTED] T_OBJECT: $3 = {flags = 21474844769, klass = 140737344040416}
$4 = {0x24, 0x24, 0x24}

(gdb) cfp + 1
CFP (count=3, addr=0x7ffff73fef90):
$5 = {pc = 0x5555567a78f8, sp = 0x7ffff72ff040, iseq = 0x7ffff26032d0, self = 140737344619296, ep = 0x7ffff72ff038, block_code = 0x0,
  __bp__ = 0x7ffff72ff040, jit_return = 0x555558c2b000}

Stack (size=0):
```

1 files changed, 2 insertions, 0 deletions

diff --git a/.gdbinit b/.gdbinit
index 074eeef4f1..5980c7f81c 100644
--- a/.gdbinit
+++ b/.gdbinit
@@ -1348,3 +1348,5 @@ define print_flags
   printf "RUBY_FL_USER17      : %s\n", ((struct RBasic*)($arg0))->flags & RUBY_FL_USER17 ? "1" : "0"
   printf "RUBY_FL_USER18      : %s\n", ((struct RBasic*)($arg0))->flags & RUBY_FL_USER18 ? "1" : "0"
 end
+
+source misc/gdb.py