internal: make CompletionItem and SourceChange consistent

Before this PR, SourceChange used a bool and CompletionItem used an enum
to signify if edit is a snippet. It makes sense to use the same pattern
in both cases. `bool` feels simpler, as there's only one consumer of
this API, and all producers are encapsulated anyway (we check the
capability at the production site).

crates/ide_completion/src/lib.rs

@@ -25,7 +25,7 @@ use crate::{completions::Completions, context::CompletionContext, item::Completi
 
 pub use crate::{
     config::CompletionConfig,
-    item::{CompletionItem, CompletionItemKind, CompletionRelevance, ImportEdit, InsertTextFormat},
+    item::{CompletionItem, CompletionItemKind, CompletionRelevance, ImportEdit},
 };
 
 //FIXME: split the following feature into fine-grained features.