Include only source files in test coverage

[mesaugat]

Additionally, require ts-node/register in nyc to leave out TS interfaces from test coverage.

[codecov-io]

Codecov Report

Merging #23 into master will increase coverage by 18.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #23       +/-   ##
==========================================
+ Coverage   51.03%   69.1%   +18.07%     
==========================================
  Files          10       5        -5     
  Lines         145     123       -22     
  Branches       19      19               
==========================================
+ Hits           74      85       +11     
+ Misses         65      31       -34     
- Partials        6       7        +1

Impacted Files	Coverage Δ
examples/express-ts/src/index.ts
examples/express-ts/src/middlewares.ts
examples/express-ts/src/service.ts
examples/express-ts/backpack.config.js
index.js
src/index.ts	59.18% <0%> (+10.53%)	⬆️

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