Tweaks to decorator generic type definitions.

[petejohanson]

I needed these changes to avoid an issue similar to the issue reported and suggested workaround in microsoft/TypeScript#17936 (comment)

Thanks!

[codecov]

Codecov Report

Merging #1 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master       #1   +/-   ##
=======================================
  Coverage   79.01%   79.01%           
=======================================
  Files           5        5           
  Lines          81       81           
  Branches       15       15           
=======================================
  Hits           64       64           
  Misses          5        5           
  Partials       12       12

Impacted Files	Coverage Δ
src/mutation.ts	87.5% <100%> (ø)	⬆️
src/action.ts	58.82% <100%> (ø)	⬆️
src/mutationaction.ts	73.68% <100%> (ø)	⬆️

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