Clean up Fetch body data

[ddbeck]

Summary

AFAICT, fetch() has always supported a request body, but only recently acquired ReadableStream bodies. This PR reorganizes the data to make this more obvious.

Test results and supporting details

This PR doesn't actually create any new data; it copies or moves it.

• api.fetch.init_body_parameter copies the support data from api.fetch
• api.Request.Request.init_body_parameter copies the support data from api.Request.Request
• api.fetch.body_readablestream moved to api.fetch.init_body_parameter.readablestream
• api.Request.request_body_readablestream moved to api.Request.Request.init_body_parameter.readablestream

Where required, I changed descriptions and added spec URLs.

Related issues

• Disambiguate fetch-request-streams feature web-platform-dx/web-features#3361 (comment)

[github-actions]

Tip: Review these changes grouped by change (recommended for most PRs), or grouped by feature (for large PRs).

[caugner]

While we're here, shouldn't we wrap all init_*_parameter features under a init_parameter feature? IIRC this is the preferred approach we had aligned on recently.

[caugner]

dito

[ddbeck]

@caugner I've pushed the big reorganization. It required creating a new root options_parameter entries to nest the properties under; I copied the data from the fetch and Request() objects respectively. It also renames init to options to follow the MDN naming convention. Good luck reviewing the diff, which is now huge. 🫠

[caugner]

@caugner I've pushed the big reorganization. It required creating a new root options_parameter entries to nest the properties under; I copied the data from the fetch and Request() objects respectively. It also renames init to options to follow the MDN naming convention. Good luck reviewing the diff, which is now huge. 🫠

Let me see if I can implement move detection into the PR Review Companion diff. This would be a super example to test it on.