Fix crash when access modifiers are used in constructor param defaults

[alangpierce]

Fixes #522

Previously, the logic to compile TS constructor param assignments walked through
all tokens in the constructor looking for access modifiers, but this can cause
problems if default values include access modifiers. Now, we mark commas
associated with the constructor and only emit assignments when we see an
open-paren or comma followed by an access modifier.

[codecov-io]

Codecov Report

Merging #531 into master will increase coverage by 0.00%.
The diff coverage is 76.92%.

@@           Coverage Diff           @@
##           master     #531   +/-   ##
=======================================
  Coverage   81.50%   81.51%           
=======================================
  Files          55       55           
  Lines        5710     5713    +3     
  Branches     1287     1289    +2     
=======================================
+ Hits         4654     4657    +3     
  Misses        766      766           
  Partials      290      290           

Impacted Files	Coverage Δ
src/util/getClassInfo.ts	85.24% <70.00%> (+0.12%)	⬆️
src/parser/traverser/lval.ts	98.82% <100.00%> (+0.02%)	⬆️
src/parser/traverser/statement.ts	83.06% <100.00%> (ø)

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