Replace concat-stream w/ node:stream/consumers

[ChristianMurphy]

Initial checklist

• I read the support docs
• I read the contributing guide
• I agree to follow the code of conduct
• I searched issues and discussions and couldn’t find anything or linked relevant results below
• I made sure the docs are up to date
• I included tests (or that’s not needed)

Description of changes

Related to unifiedjs/unified-engine#92 and unifiedjs/unified-engine#93.

The MarkdownAsync tests now pipe renderToPipeableStream output into a PassThrough and read it with text() from node:stream/consumers.
Before, they collected it with concat-stream inside a new Promise.
concat-stream leaves devDependencies, and the shared TextDecoder in test.jsx goes too, since nothing else used it.
An error while rendering, such as a throwing rehype plugin, now rejects the test through await text().

Only the tests change.
The published package stays the same.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (5088035) to head (73e9721).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #946   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines         1787      1778    -9     
  Branches       123       119    -4     
=========================================
- Hits          1787      1778    -9     

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Thanks!