[Feature Request] Include an optional 'source id' string to terrain/templates

[PepijnMC]

I have been trying to implement stacking rules for DnD5e but the system takes a very simple and broad approach with only 1 major rule:

• Movement penalties stack unless they are from the same source.

The source can be difficult terrain (it's a proper game term in 5e), crawling, swimming/climbing without having the proper movement speed for it, a specific spell (some spells opt to define this as difficult terrain in which case that is the source, but not every spell does this), etc.

As you can imagine, although the rule is simple, trying to apply this automatically with Enhanced Terrain is not really possible at the moment. An optional 'source id' string that can be defined for all terrain, templates, and especially within spells would allow my users to have control over what should stack with what instead of me awkwardly implementing them.