Reduce bundle size of parser-postcss.js

[sosukesuzuki]

Based on #12203

Description

Before:

$ node ./scripts/build/build.mjs --file=parser-postcss.js --print-size --report
 Building packages 
parser-postcss.js...............................................238 kB    DONE  

After:

$ node ./scripts/build/build.mjs --file=parser-postcss.js --print-size --report
 Building packages 
parser-postcss.js...............................................193 kB    DONE  

Checklist

• I’ve added tests to confirm my change works.
• I’ve read the contributing guidelines.

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