Goals: Schedule multi month forecasting (#1452)

References discord discussion starting here:
https://discord.com/channels/937901803608096828/940290142579605514/1133523705063030824

Currently the schedule keyword won't fill any future budget cells if the
category balance already satisfies the schedule. This PR is an attempt
to improve the behavior by allowing budget fills regardless of the
category balance.

This is a drastic rewrite of the schedule keyword. Though I've tried not
to have any regressions, it is possible because of how different the
logic is. I've tested compounding using a simple template, so a small
change in the 'by' keyword was also made.