Add type declaration for singleton field

### Problem

Since singleton is a binary flag similar to comp, it should return results the same way comp does. As of now, they do differ:

```
% beet ls comp:1|wc -l
15163

% beet ls comp:true|wc -l
15163

% beet ls singleton:1|wc -l
0

% beet ls singleton:true|wc -l
14345

Start with the beet ls field-query examples in the issue and trace how comp and singleton values are parsed, locating the type declaration for singleton. Done means singleton:1 and singleton:true return the same results as comp:1 and comp:true.

Written by the indexing model from the issue text.