feat: Config plus improvements (again...)

[ThisAMJ]

• Primitive svar completion for svar_get, svar_substr, svar_[no_]persist, svar_capture, svar_from_cvar, and operations
• Remove the single-argument limit on svar_set (e.g. svar_set a b c => a = b c)
• sar_on_{x}_list for debugging / curiosity. arguably could add a index to the list and sar_on_{x}_remove <i> but that's too fancy
• $0/$+0 in expansion -- quite dubious, feel free to drop but I don't see the harm
• Re-order the code to be feature-contiguous
• Floating point svar math!! Adds svar_round, svar_floor, svar_ceil, and svar_abs (the last of which is pretty superfluous)

[ThisAMJ]

for back-compat im going to put floating math behind different operations e.g. svar_fadd people_who_care 0.5