use Object.create for each block child contexts

[Rich-Harris]

This results in slightly better performance and lower memory usage when creating large lists

[codecov-io]

Codecov Report

Merging #1422 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #1422   +/-   ##
=======================================
  Coverage   91.07%   91.07%           
=======================================
  Files         122      122           
  Lines        4484     4484           
  Branches     1400     1400           
=======================================
  Hits         4084     4084           
  Misses        158      158           
  Partials      242      242

Impacted Files	Coverage Δ
src/compile/nodes/EachBlock.ts	97.95% <100%> (ø)	⬆️

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