add designspace font attributes

[typemytype]

see discussion https://discord.com/channels/1052516637489766411/1384544391699628112

setting font.designspaces with a path or an DS object

getting a list of designspace objects only if the path exists and optionally check if the font is still a source of that designspace.

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Add options to get the next font, heavier. Not sure how this could work as a font source can be in multiple designspaces.

FYI @connordavenport

[LettError]

I like this idea, but don't know where to implement it?