toplevel debugging improvements

[aviatesk]

• set task_local_storage for toplevel debugging
• enable "step-to-selected-line" for toplevel debugging: currently results in error

closes JunoLab/Juno.jl#569

[codecov]

Codecov Report

Merging #329 into master will increase coverage by 0.06%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #329      +/-   ##
==========================================
+ Coverage   54.94%   55.00%   +0.06%     
==========================================
  Files          42       42              
  Lines        2619     2616       -3     
==========================================
  Hits         1439     1439              
+ Misses       1180     1177       -3     

Flag	Coverage Δ
#unittests	55.00% <0.00%> (+0.06%)	⬆️

Impacted Files	Coverage Δ
src/debugger/stepper.jl	1.38% <0.00%> (+0.01%)	⬆️

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