Add "text" destination type, for JavaScript text imports

[eemeli]

This change is to support the changes in whatwg/html#11933, by adding text as a destination type.

• At least two implementers are interested (and none opposed):
  • Chromium
  • Gecko
  • Deno
  • Node.js
  • …
• Tests are written and can be reviewed and commented upon at:
  • Add tests for text modules web-platform-tests/wpt#56812
• Implementation bugs are filed:
  • Chromium: …
  • Gecko: …
  • WebKit: …
  • Deno (not for CORS changes): Stabilize bytes and text imports denoland/deno#29904
• MDN issue is filed: …
• The top of this comment includes a clear commit message to use.

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[annevk]

This looks reasonable to me, modulo table nit.

[styfle]

@eemeli @annevk Can this be merged now? I'm going to make a similar PR for import bytes and I'm hoping to avoid merge conflicts.