Minor fixes to the build system

[bring-shrubbery]

• Fixed incorrect type declaration path
• Added gzip and terser plugins to rollup.config.js
• Replaced lines of code badge with gzipped size badge

[codecov-commenter]

Codecov Report

Merging #17 (36b5abd) into main (c116558) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #17   +/-   ##
=======================================
  Coverage   93.47%   93.47%           
=======================================
  Files           3        3           
  Lines          46       46           
  Branches       14       14           
=======================================
  Hits           43       43           
  Misses          3        3           

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