[Magiclysm] Extend custom sidebar to include mana, max mana, and mana graph

[wapcaplet]

Summary

Mods "[Magiclysm] Extend custom sidebar to include mana, max mana, and mana graph"

Purpose of change

Enable the custom sidebar to display mana level, but only when Magiclysm mod is enabled.

Fix #54297

Describe the solution

• Allow widget::load to read an "extends" object's nested "widgets"
• Implement a max_mana widget variable (since max mana is indeed variable)
• Add data/mods/Magiclysm/ui/sidebar.json with mana widgets, layouts, and sidebar extensions

Describe alternatives you've considered

The generic_factory seems like it natively supports the use of "extends" and "delete", but I couldn't get it to work without adding an explicit "extends" handler inside widget::load. I expect there's a cleaner and more generalized way to do this, and would be thrilled if anyone can tell me how. Done, thanks @anothersimulacrum for digging up the deets.

Testing

Additional context

Positioned mana sections of custom sidebar at the top for easier screenshotting:

How the new widget sections appear in the sidebar:

The graph is the first one to use the "pool" method of symbol filling - notice how the full bar depletes from # to =, then down to - and finally .:

Nearly out of mana:

Recharged some mana, then cast synaptic stimulation - maximum mana increased:

A few examples with both "Mana Graph" and "Mana Pool":

[kevingranade]

Neat you reinvented sparklines.