Add `:CodefmtInfo` command to report diagnostics to help users check config / diagnose problems

We should have a command called something like `:CodefmtInfo` that can dump a report on how it's configured wrt the current context and maybe the last known invocation.

For example:
```vim
:CodefmtInfo
```
```
Current buffer: foo.py
Autoformat [enabled|disabled] [using formatter yapf]
Selected formatter: [yapf]
All available formatters: yapf, black, isort, autopep8

Last invocation at [TIMESTAMP]:
Executed command: yapf -blah ...
Exit code: [0|...]
[Stderr: ...]
```