fix: remove ElementType from union in MergeWithAs

[kris-ellery]

Fixes #337.

[codecov]

Codecov Report

Merging #340 (e037546) into main (7291314) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #340   +/-   ##
=======================================
  Coverage   99.69%   99.69%           
=======================================
  Files          11       11           
  Lines         665      665           
  Branches       58       58           
=======================================
  Hits          663      663           
  Misses          2        2           

Flag	Coverage Δ
preact	100.00% <ø> (ø)
react	100.00% <100.00%> (ø)
solid	100.00% <ø> (ø)
vue	98.83% <ø> (ø)

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Impacted Files	Coverage Δ
packages/react/src/forwardRef.tsx	100.00% <100.00%> (ø)

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