fix: ensure no scripts will be overwritten, when providing case aliases

[SleepWalker]

What:

Fixed a bug from #172.

How:

There was no check for existence of the scripts during generation of aliases for scripts in camel/kebab cases (kebab-and-camel-casify.js). So if one script after case conversion yields the key that is already taken by the other script in config it will be overwritten.

Now we will check whether alias key is not taken by the other script.

Checklist:

• Documentation N/A
• Tests
• Ready to be merged
• Added myself to contributors table

[codecov]

Codecov Report

Merging #173 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #173   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          12     12           
  Lines         382    382           
  Branches       92     92           
=====================================
  Hits          382    382

Impacted Files	Coverage Δ
src/kebab-and-camel-casify.js	100% <100%> (ø)	⬆️

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[kentcdodds]

Looks good. Thanks!