Provide tools to analyze the files in a database.

Recently, I tried to use CodeQL to analyze my project, and all the steps succeeded without any warnings. However, after running some queries I realized that it was missing most of the files.

As a user, it would be nice to be able to list the files that are included in the database with just a command.

After talking with @adityasharad , he suggested using a simple query that just lists all the files:

```
/**
* @description "List all the files"
* @id debug.listfiles
* @kind problem
* @name list-files
* @tags maintanability
* @problem.severity recommendation
**/
import cpp

from File f
select f, "This file is in the repo"
```

It would be nice to have something like that built into the application.

Start with the CodeQL CLI and the simple `debug.listfiles` query shown in the issue; compare its output with the files included in the database. Define a built-in command that lists those files and verify that it exposes files currently omitted from analysis.

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