Add --nobuild flag to gulp serve
When using a monorepo manager, like for example @microsoft/rush, the rush build command bundles all projects which are referenced in the monorepo - including SPFx projects.
When running gulp serve in (one of) the SPFx project(s), the bundle gulp command rebuilds the project. This is an unnecessary step, which would be nice if we could disable.
Add a --nobuild flag support to the gulp serve command so we can start the local web server without rebuilding the SPFx code.