Add the number of processed files to the output

On success, the action prints:
```
Codespell found no problems
```
However it will print the same message if it did not check any files at all due to, for example, an issue in the path specification. It would be nice if it also printed how many files it checked to avoid any doubts, e.g.:
```
Codespell found no problems in 123 files
```