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/src/lib.rs

@@ -97,7 +97,6 @@ pub use ide_assists::{
 };
 pub use ide_completion::{
     CompletionConfig, CompletionItem, CompletionItemKind, CompletionRelevance, ImportEdit,
-    InsertTextFormat,
 };
 pub use ide_db::{
     base_db::{