Unified project template conventions and mitigating accidental inclusion in WebAssembly build

[SteveSandersonMS]

We need to update the new project template and SDK rules so that:

• There are good conventions for including/excluding things in the Server and WebAssembly builds based on:
  • Directory
  • Filename
  • #if
• We help developers not to include code or other assets like config in the WebAssembly compilation by mistake
  • For example, we might only include .cs files from a WebAssembly or Shared directory
  • For project/package references, we might have special metadata for "server", "webassembly", and "shared". Then if you add any project/package reference and it doesn't have this metadata (which is the case by default) then:
    • During development this logs a warning
    • During publish it's an error
    • ... so that developers aren't prevented from following existing tutorials (especially important for learners) but they do get a reasonable warning that they have to take some action before publishing to express a clear intent

[danroth27]

Is this a duplicate of #46398?

[SteveSandersonMS]

Yes, closing as duplicate of that.