Allow to enable/disable JS asynchronous generation

[soywiz]

Since this affects negatively to performance (specially before detecting synchronous paths is implemented), this should be opt-in.

For example. Benchmark:

TOTAL time: 6615.0 <--- without asynchronous
TOTAL time: 30309.0 <--- with asynchronous without optimizations (5x slower)

[codecov[bot]]

Codecov Report

Merging #277 into master will increase coverage by 0.06%. The diff coverage is 84.61%.

@@             Coverage Diff              @@
##             master     #277      +/-   ##
============================================
+ Coverage     17.86%   17.93%   +0.06%     
  Complexity      386      386              
============================================
  Files           314      314              
  Lines         20809    20825      +16     
  Branches       4495     4501       +6     
============================================
+ Hits           3718     3734      +16     
- Misses        16407    16408       +1     
+ Partials        684      683       -1

Impacted Files	Coverage Δ	Complexity Δ
...nsc-rt-core/src/com/jtransc/io/JTranscConsole.java	0% <ø> (ø)	0 <0> (ø)	:arrow_down:
...transc-rt-core/src/com/jtransc/JTranscWrapped.java	0% <ø> (ø)	0 <0> (ø)	:arrow_down:
...-core/src/com/jtransc/text/JTranscStringTools.java	0% <ø> (ø)	0 <0> (ø)	:arrow_down:
...core/src/com/jtransc/gen/common/CommonGenerator.kt	0% <0%> (ø)	0 <0> (ø)	:arrow_down:
jtransc-gen-js/src/com/jtransc/gen/js/JsTarget.kt	86.81% <89.18%> (+1%)	4 <0> (ø)	:arrow_down:

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