Address review feedback: rename constructors, unbox FormDataEntryValue, reuse MessageEvent types, simplify tests

[jderochervlk]

Motivation

• Improve API clarity by replacing numeric/ambiguous overloaded constructors with descriptive names for MediaStream.
• Simplify and stabilize the public API surface by consolidating types and avoiding redundant local definitions for MessageEvent.
• Make test fixtures concise and idiomatic by removing unnecessary explicit type annotations and unused aliases.
• Follow reviewer guidance to prefer unboxed lightweight variants for FormData entry values for ergonomics.

Description

• Rename MediaStream overloaded constructors in packages/MediaCaptureAndStreams/src/MediaStream.res from make2/make3 to makeFromMediaStream/makeFromMediaStreams for clarity.
• Replace the alias to Types.formDataEntryValue with a local @unboxed variant API in packages/Fetch/src/FormDataEntryValue.res and expose fromString, fromFile, and a decode helper.
• Reuse the shared MessageEvent record/init types from packages/WebSockets/src/Types.res by aliasing them in packages/WebSockets/src/MessageEvent.res instead of re-defining the record shape.
• Simplify test fixtures by removing redundant explicit type annotations in tests/FetchAPI/Request__test.res and tests/FetchAPI/Response__test.res, and drop unused alias bindings in tests/Global__test.res.
• Remove numeric overload bindings in packages/DOM/src/Window.res (alert2, scroll2, scrollTo2, scrollBy2) and keep the descriptive bindings such as alertWithMessage, scrollXY, scrollToXY, and scrollByXY.
• Each change was committed with Co-authored-by: Codex <codex@openai.com> in the commit metadata.

Testing

• Ran npm test, which runs the build step (rescript), and the run failed due to an existing missing module alias error: the build errored with a missing module alias ReadableStream-WebApiFile reported from packages/File/src/Blob.res.
• The earlier rescript compilation parsed sources and compiled modules before the failure, indicating the changes themselves did not introduce new compiler errors prior to hitting the unrelated missing-alias issue.

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Codex Task