Add config to unconditionally prefer core imports over std

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

crates/ide-completion/src/config.rs

@@ -17,6 +17,7 @@ pub struct CompletionConfig {
     pub callable: Option<CallableSnippets>,
     pub snippet_cap: Option<SnippetCap>,
     pub insert_use: InsertUseConfig,
+    pub prefer_core: bool,
     pub snippets: Vec<Snippet>,
 }