Index messages with attachments using a boolean

When indexing message attachment metadata using numeric indexes such as:

```javascript
{
  conversationId: '+12223334455',
  received_at: 123,
  attachments: […],
  numAttachments: 2,
},
{
  conversationId: '+12223334455',
  received_at: 456,
  attachments: [],
  numAttachments: 0,
}
{
  conversationId: '+12223334455',
  received_at: 789,
  attachments: [],
  numAttachments: 1,
}
```

It creates an index as follows:

```
[conversationId, received_at, numAttachments]
['+12223334455', 123, 2]
['+12223334455', 456, 0]
['+12223334455', 789, 1]
```

This means a query such as…

```
lowerBound: ['+12223334455', 0,                1               ]
upperBound: ['+12223334455', Number.MAX_VALUE, Number.MAX_VALUE]
```

…will return all three original entries because they span the `received_at`
from `0` through `Number.MAX_VALUE`. One workaround is to index booleans using
`1 | undefined` where `1` is included in the index and `undefined` is not, but
that way we lose the ability to query for the `false` value. Instead, we flip
adjust the index to `[conversationId, hasAttachments, received_at]` and can
then query messages with attachments using

```
[conversationId, 1 /* hasAttachments */, 0                /* received_at */]
[conversationId, 1 /* hasAttachments */, Number.MAX_VALUE /* received_at */]
```

ts/types/IndexedDB.ts

@@ -0,0 +1,12 @@
+/**
+ * @prettier
+ */
+
+// IndexedDB doesn’t support boolean indexes so we map `true` to 1 and `false`
+// to `0`.
+// N.B. Using `undefined` allows excluding an entry from an index. Useful
+// when index size is a consideration or one only needs to query for `true`.
+export type IndexableBoolean = 1 | 0;
+
+export const toIndexableBoolean = (value: boolean): IndexableBoolean =>
+  value ? 1 : 0;