Fix calling of npx

[bustbr]

According to the NPX documentation for the syntax we're using the -- is not supported (or rather has no special meaning) and everything (including any --) after the package name will be passed on to the command. This PR tries to fix #11 by simply dropping the injected --.

We also add a unit test for building the npx command.

[codecov[bot]]

Codecov Report

Merging #18 (51196b0) into main (e3a8978) will increase coverage by 0.69%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #18      +/-   ##
==========================================
+ Coverage   74.64%   75.34%   +0.69%     
==========================================
  Files          12       13       +1     
  Lines         284      292       +8     
==========================================
+ Hits          212      220       +8     
  Misses         72       72              

Impacted Files	Coverage Δ
pyright/cli.py	92.85% <100.00%> (+0.54%)	:arrow_up:
tests/test_cli.py	100.00% <100.00%> (ø)

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[RobertCraigie]

Going to add support for windows before merging this PR, thanks for your contributions!