Add dual_set_type

[blegat]

Needed for implementing supports_constraint for Dualization, see jump-dev/Dualization.jl#77

[codecov-io]

Codecov Report

Merging #1002 into master will decrease coverage by 0.01%.
The diff coverage is 89.47%.

@@            Coverage Diff             @@
##           master    #1002      +/-   ##
==========================================
- Coverage   95.19%   95.18%   -0.02%     
==========================================
  Files          99       99              
  Lines       11166    11184      +18     
==========================================
+ Hits        10630    10646      +16     
- Misses        536      538       +2

Impacted Files	Coverage Δ
src/sets.jl	95.28% <89.47%> (-1.31%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 49180c6...29e9ea2. Read the comment docs.