Use the last line of the npm pack command

[jtpio]

To fetch the name of the npm tarball, which should be the last statement of the npm pack output.

As noticed in jupyterlab/extension-cookiecutter-ts#162 (comment)

[codecov-commenter]

Codecov Report

Merging #37 (3f99e3a) into master (9129099) will decrease coverage by 0.11%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master      #37      +/-   ##
==========================================
- Coverage   88.51%   88.39%   -0.12%     
==========================================
  Files          15       15              
  Lines        1715     1724       +9     
  Branches      205      208       +3     
==========================================
+ Hits         1518     1524       +6     
- Misses        122      124       +2     
- Partials       75       76       +1     

Impacted Files	Coverage Δ
jupyter_releaser/lib.py	75.89% <62.50%> (-0.48%)	⬇️
jupyter_releaser/__init__.py	100.00% <100.00%> (ø)
jupyter_releaser/npm.py	80.91% <100.00%> (+0.29%)	⬆️
jupyter_releaser/util.py	85.00% <100.00%> (+0.10%)	⬆️

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[blink1073]

Thanks!