[lldb] Use __lldb_init_module instead of "if lldb.debugger" idiom

Update examples and docs to demonstrate using `__lldb_init_module` instead of
the idiom that checks for `lldb.debugger` at the top-level.

```
if __name__ == '__main__':
    ...
elif lldb.debugger:
    ...
```

Is replaced with:

```
if __name__ == '__main__':
    ...

def __lldb_init_module(debugger, internal_dict):
    ...
```

This change is for two reasons. First, it's generally encouraged not to only
use the convenience singletons (`lldb.{debugger,process,target,etc}`)
interactively from the `script` command. Second, there's a bug where
registering a python class as a command (using `command script add -c ...`),
result in the command not being runnable. Note that registering function-backed
commands does not have this bug.

Differential Revision: https://reviews.llvm.org/D117237