Breaking changes in other repos

[nex3]

This is a meta-issue that tracks changes in other repositories that will break an API that matches Dart Sass's version number. These issues are logically part of Milestone 2.0.0, but that milestone only applies to this repo so this meta-issue acts as a link from it to them.

• Improve generated Java code embedded-protocol#95
• Use proto3 optional fields sass#3575
• Improve unit strictness for function parameters sass#3374
• Deprecate and remove support for default export embedded-host-node#229