Replace Deno.platform usages

[justjavac]

Deno.platform was deprecated and it was reorganized into Deno.build at denoland/deno#1879 . This PR replaces Deno.platform usages with Deno.build.

[codecov-io]

Codecov Report

Merging #53 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master      #53   +/-   ##
=======================================
  Coverage   59.11%   59.11%           
=======================================
  Files           6        6           
  Lines         362      362           
  Branches       91       85    -6     
=======================================
  Hits          214      214           
  Misses        145      145           
  Partials        3        3

Impacted Files	Coverage Δ
src/node.ts	71.42% <0%> (ø)	⬆️

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[egoist]

🎉 This PR is included in version 6.5.3 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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