fix: add missing main in package.json

[hieven]

Which problem is this PR solving?

• Add missing main to package.json

Short description of the changes

• Missing main in package.json makes developer unable to do require("@opentelemetry/shim-opentracing");

[codecov-io]

Codecov Report

Merging #577 into master will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #577      +/-   ##
=========================================
- Coverage   90.54%   90.5%   -0.05%     
=========================================
  Files         156     155       -1     
  Lines        8019    8004      -15     
  Branches      692     704      +12     
=========================================
- Hits         7261    7244      -17     
- Misses        758     760       +2

Impacted Files	Coverage Δ
...ages/opentelemetry-plugin-http/test/utils/utils.ts	33.33% <0%> (-26.67%)	⬇️
...res/opentelemetry-plugin-pg/test/assertionUtils.ts	96.29% <0%> (-3.71%)	⬇️
...core/src/context/propagation/BinaryTraceContext.ts	96.72% <0%> (-1.62%)	⬇️
...opentelemetry-base/test/resources/resource.test.ts	100% <0%> (ø)	⬆️
...entelemetry-exporter-jaeger/test/transform.test.ts	100% <0%> (ø)	⬆️
.../opentelemetry-core/src/trace/spancontext-utils.ts	100% <0%> (ø)	⬆️
...pe-async-hooks/test/AsyncHooksScopeManager.test.ts	100% <0%> (ø)	⬆️
.../opentelemetry-exporter-jaeger/test/jaeger.test.ts	100% <0%> (ø)	⬆️
...telemetry-scope-base/test/NoopScopeManager.test.ts	100% <0%> (ø)	⬆️
...opentelemetry-core/src/trace/globaltracer-utils.ts	100% <0%> (ø)	⬆️
... and 7 more