fix: replace any type with unknown

[kristw]

No description provided.

[codecov]

Codecov Report

Merging #93 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #93   +/-   ##
=======================================
  Coverage   13.18%   13.18%           
=======================================
  Files           9        9           
  Lines          91       91           
  Branches       18       18           
=======================================
  Hits           12       12           
  Misses         79       79

Impacted Files	Coverage Δ
packages/react-vega/src/utils/isFunction.ts	100% <ø> (ø)	⬆️
packages/react-vega/src/createClassFromSpec.tsx	0% <ø> (ø)	⬆️
packages/react-vega/src/Vega.tsx	0% <ø> (ø)	⬆️

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