Script wasn't working on my machine with GDB 16.2
Main issues
- `gdb.types` wasn't imported, leading to import error on initial load
- dollar function didn't work with the new mangling scheme
Fixes them, also updates the test script to work with some new mangling
changes.
Test evidence
(cherry picked from commit e0a4876981)
* Fix custom objects, fix handling of nil variables
* Rename because lldb complains about dashes in filename
* Add example of how to use custom type formatting from Nim
* Add synthetic objects with dynamic fields and add an example
