Add `has()` and `hasMany()`

[vweevers]

Per the feature request of https://github.com/Level/community/issues/106. Will add two methods:

await db.put('love', 'u')
await db.has('love') // true
await db.hasMany(['love', 'hate']) // [true, false]

Tasks:

• [x] level-supports (6.2.0): https://github.com/Level/supports/pull/33
• [x] abstract-level (3.0.0): https://github.com/Level/abstract-level/pull/96
• [x] memory-level (3.0.0): https://github.com/Level/memory-level/pull/18
• [x] classic-level (3.0.0): https://github.com/Level/classic-level/pull/111
• [x] browser-level (3.0.0): https://github.com/Level/browser-level/pull/25
• [x] level (10.0.0): bump dependencies
• [ ] many-level: implement has() and hasMany()
• [ ] rave-level: implement has() and hasMany()