fix(cron): support a single named month in expressions

[ASTROWwwW]

Problem

A single named month (e.g. jan) never runs. parseCron returns it as the string "1", which getTimeUnit can't match (not a step/range/list) → returns false → the task is stopped. Numeric (1) and lists (jan,feb) work fine.

Reproduce

lib.cron.new('* * * jun *', fn) -- never runs (named, current month)
lib.cron.new('* * * 6 *',   fn) -- runs (numeric)

Fix

Return a number for a single parsed month so it takes the numeric path: