Bump `package:web` to 1.0.0

[navaronbracke-endare]

This PR bumps the web dependency to 1.0.0

It also bumps the minimum Flutter version to 3.22.0, which ships with Dart 3.4.0 That Dart version is the minimum needed for the new package:web version.

It also addresses the following points:

• the structs in the ffi types for windows should be marked base, sealed or final, due to a Dart language change. I made them final.
• the web version of this plugin had two open TODO's that needed Dart 3.4, these have been fixed.
• the web version also had a redundant static variable platform, that wasn't used except for setting up the web platform implementation. This has been removed, as users should not rely on FilePickerWeb.platform, but rather on FilePicker.platform

Fixes #1571 Closes https://github.com/miguelpruivo/flutter_file_picker/pull/1575 Closes https://github.com/miguelpruivo/flutter_file_picker/pull/1566

For clarification, this is the corporate account of @navaronbracke I'm currently away from my other machine but wanted to land this fix ASAP.

[navaronbracke]

Due to the urgency for the dependency bump, I'm going to merge this PR, so users can start migrating as well.

Most changes are internal cleanup anyway, and if users were using FilePickerWeb.platform, they were doing it wrong anyway (which is addressed by the comment in the changelog)