Improve ease of selecting a database on the filesystem

[aeisenberg]

https://github.com/github/codeql-coreql-team/issues/70

Slightly confusing that a database is a folder that can be navigated deeper into.

We could allow the user to select the (suitably renamed) .dbinfo file instead.

[aeisenberg]

I think the easiest and most useful thing to do here is to apply the following heuristics:

1. If the selected URI to add is a file, choose the containing directory
2. If the selected URI is a directory matching db-*, choose the containing directory
3. choose the current directory

This will allow someone to select the deeply nested .dbscheme file, or the .dbinfo file, or any number of other sibling files. Being liberal in what we accept here is not dangerous to the user.

[dbartol]

https://code.visualstudio.com/api/references/vscode-api#OpenDialogOptions claims that allowing the dialog to open both files and folders doesn’t work on Windows.
Is there a specific file extension we can filter on?

[aeisenberg]

Currently, we specify that only folders can be opened. So, it's not technically possible at this point for a user to select the .dbscheme file, but it is still possible for the user to select the db-* folder. So, there really is no file extension we can use. The only feature is that if someone accidentally chooses the db-* folder, the extension should handle that.

So, I'll just leave #1 unimplemented since it's not possible.