Add support for num_jobs builtin function

[terror]

Adds the num_jobs() nullary function (just master, casey/just#3537) to the builtin catalog and completion/hover surfaces.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.08%. Comparing base (254d82e) to head (d86d2bc).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #457      +/-   ##
==========================================
+ Coverage   95.03%   95.08%   +0.04%     
==========================================
  Files          59       59              
  Lines       13133    13262     +129     
==========================================
+ Hits        12481    12610     +129     
  Misses        652      652              

Flag	Coverage Δ
unit	95.08% <100.00%> (+0.04%)	⬆️

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