Rename purge to drop in TS types

[danilofuchs]

Since pg-boss 10, purge was renamed to drop

Also, monitor and supervise were not declared

[coveralls]

coverage: 100.0%. remained the same
when pulling d6038f7 on danilofuchs:patch-1
into 625779c on timgit:master.

[timgit]

For now, dropping purge is the only appropriate change. supervise and monitor are not externally exposed. drop will become obsolete soon. Also, these are not documented.