pkg: add lockfile

[braydonf]

See: https://github.com/bcoin-org/bcoin/issues/619

[codecov-io]

Codecov Report

Merging #116 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #116   +/-   ##
=======================================
  Coverage   55.48%   55.48%           
=======================================
  Files         109      109           
  Lines       27169    27169           
  Branches     4496     4496           
=======================================
  Hits        15074    15074           
  Misses      12095    12095

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[braydonf]

cc: @tuxcanfly and @nodar-chkuaselidze please verify that these are your public keys for signing release tags.

[tuxcanfly]

Verified my key for [email protected].