Implement per-action base weights

[mkremins]

Some actions are just inherently more narratively significant than others, and should be prioritized as suggestions whenever available. As a first pass at enabling this behavior, we should:

• Allow action authors to add an integer weight to every action definition.
• Assign a default weight (1?) at action registration time if none is specified.
• Adjust action suggestion sorting, which currently only respects author goals, to consider action base weights as well – maybe by summing or multiplying them with author goal weights.