Replace hardcoded .net assembly references with DependencyContext-based resolution

ScriptLoader.GetDefaultReferences() hardcodes ~40 individual System.* assembly references for Roslyn script compilation. When a user's build script needs an assembly not in this
list, it fails with cryptic CS0012/CS0246 compilation errors.

Use DependencyContext.Default.CompileLibraries to resolve compile-time references from the host application's .deps.json at runtime. This gives the full set of framework references the host was compiled against, without hardcoding.