Add manifest to levelup and abstract-leveldown implementations

[vweevers]

Background and discussion: #42.

Add manifest to:

• [x] abstract-leveldown
  • [x] PR or commit: https://github.com/Level/abstract-leveldown/pull/351
  • [x] Release: 6.2.0
• [x] encoding-down
  • [x] PR or commit: https://github.com/Level/encoding-down/pull/93
  • [x] Release: 6.3.0
• [x] leveldown
  • [x] PR or commit: https://github.com/Level/leveldown/pull/685
  • [x] Release: 5.4.0
• [x] deferred-leveldown
  • [x] Related: https://github.com/Level/deferred-leveldown/issues/35
  • [x] PR or commit: https://github.com/Level/deferred-leveldown/pull/79
  • [x] Release: 5.3.0
• [x] levelup
  • [x] PR or commit: https://github.com/Level/levelup/pull/678
  • [x] Release: 4.3.2
• [x] memdown
  • [x] PR or commit: https://github.com/Level/memdown/pull/198
  • [x] Release: 5.1.0
• [x] level-packager
  • [x] PR or commit: https://github.com/Level/packager/commit/cd22e66
  • [x] Release: 5.1.0
• [x] level-js
  • [x] PR or commit: https://github.com/Level/level-js/pull/183
  • [x] Release: 5.0.0
• [x] level
  • [x] PR or commit: https://github.com/Level/level/commit/97cb31cc06a2586cd0baa7e65ae9451841da7734, https://github.com/Level/level/pull/162
  • [x] Release: 6.0.0
• [x] level-mem: 6.0.0
• [x] rocksdb
  • [x] PR or commit: https://github.com/Level/rocksdb/commit/5eee57739b8c1909585da1734a25ce2383688755
  • [x] Release: 5.0.0
• [x] level-rocksdb
• [x] subleveldown
  • [x] PR or commit: https://github.com/Level/subleveldown/commit/f7ee1e322276e214bbbb6db0c2034e669f35502b
  • [x] Release: 6.0.0
• [x] multileveldown (and by extension, level-party)
  • [x] PR or commit: https://github.com/Level/multileveldown/commit/b331481d7470958da5ef88bcddc292e8b45e7df6
  • [x] Release: 5.0.0
• [x] leveldown-hyper and level-hyper (slated to be deprecated)