feat: Streaming file uploads (#14775)

feat: Streaming file uploads (#14775)

* start

* pass in form_dat

* serialization

* start deserializer

* finished? deserializer

* upload progress via XHR

* simplify file offsets, sort small files first

* don't cache stream

* fix scoped ids

* tests

* re-add comment

* move location & pathname back to headers

* skip test on node 18

* changeset

* polyfill file for node 18 test

* fix refreshes

* optimize file offset table

* typo

* add lazyfile tests

* avoid double-sending form keys

* remove xhr for next PR

* fix requests stalling if files aren't read

* Update new-rivers-run.md

* encode text before determining length

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Co-authored-by: Rich Harris <rich.harris@vercel.com>