Create versioning policy for emscripten

We should decide if we even plan to use the major or minor version numbers again in emscripten or if we want to simply continue to use patch versions.

If we are going to drop major and minor bumps should we just move so a single number the chrome and firefox do it?

Split out from discussion in https://github.com/emscripten-core/emscripten/pull/18191#issuecomment-1480173774