Ensure build folder is cleared properly on each run

[guybedford]

This will remove the current typescript/typescript.js which was left behind in the last build which will reduce the package size by 3MB!

[codecov-io]

Codecov Report

Merging #246 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #246   +/-   ##
=======================================
  Coverage   69.25%   69.25%           
=======================================
  Files          16       16           
  Lines         670      670           
=======================================
  Hits          464      464           
  Misses        206      206

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