Add config to unconditionally prefer core imports over std

Fixes https://github.com/rust-lang/rust-analyzer/issues/12979

crates/ide-diagnostics/src/lib.rs

@@ -150,6 +150,7 @@ pub struct DiagnosticsConfig {
     pub expr_fill_default: ExprFillDefaultMode,
     // FIXME: We may want to include a whole `AssistConfig` here
     pub insert_use: InsertUseConfig,
+    pub prefer_core: bool,
 }
 
 impl DiagnosticsConfig {
@@ -170,6 +171,7 @@ impl DiagnosticsConfig {
                 group: false,
                 skip_glob_imports: false,
             },
+            prefer_core: false,
         }
     }
 }