perf: 3x to 10x improvement for string casing methods

[faustbrian]

Summary

Increased the performance of various string methods from anywhere between 3x and 10x by using a different regular expression that covers most use-cases and use the String(...) method for the toString method.

Checklist

• Documentation (if necessary)
• Tests (if necessary)
• Ready to be merged

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@46a368a). Click here to learn what that means.
The diff coverage is 100%.

@@           Coverage Diff           @@
##             master    #20   +/-   ##
=======================================
  Coverage          ?   100%           
=======================================
  Files             ?    108           
  Lines             ?    531           
  Branches          ?     36           
=======================================
  Hits              ?    531           
  Misses            ?      0           
  Partials          ?      0

Impacted Files	Coverage Δ
src/internal/index.ts	100% <ø> (ø)
src/internal/compound-words.ts	100% <100%> (ø)
src/to-string.ts	100% <100%> (ø)
src/lower-case.ts	100% <100%> (ø)
src/at.ts	100% <100%> (ø)
src/to-upper.ts	100% <100%> (ø)
src/to-lower.ts	100% <100%> (ø)
src/words.ts	100% <100%> (ø)

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