fix(exporter-zipkin): add main property - entry point to the module

[mayurkale22]

Which problem is this PR solving?

• Closes fix(exporter-zipkin): add main property - entry point to the module  #287

• Add missing main property - entry point to the Zipkin exporter module.

[codecov-io]

Codecov Report

Merging #289 into master will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #289      +/-   ##
==========================================
+ Coverage   98.86%   98.89%   +0.03%     
==========================================
  Files          78       78              
  Lines        3247     3163      -84     
  Branches      211      213       +2     
==========================================
- Hits         3210     3128      -82     
+ Misses         37       35       -2

Impacted Files	Coverage Δ
.../opentelemetry-exporter-jaeger/test/jaeger.test.ts	100% <0%> (ø)	⬆️
...ges/opentelemetry-exporter-jaeger/src/transform.ts	100% <0%> (ø)	⬆️
...entelemetry-exporter-jaeger/test/transform.test.ts	100% <0%> (ø)	⬆️
...ckages/opentelemetry-core/test/common/time.test.ts	100% <0%> (ø)	⬆️
packages/opentelemetry-core/src/common/time.ts	98.21% <0%> (+1.6%)	⬆️
...ckages/opentelemetry-exporter-jaeger/src/jaeger.ts	92% <0%> (+2.34%)	⬆️