fix: specify type declaration file more precisely

[danielroe]

(In another project) rollup struggled to recognise the types for defu without this change.

[codecov]

Codecov Report

Merging #15 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #15   +/-   ##
=======================================
  Coverage   95.23%   95.23%           
=======================================
  Files           1        1           
  Lines          21       21           
  Branches        6        6           
=======================================
  Hits           20       20           
  Misses          1        1           

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[pi0]

Thanks