Move code to the appropriate layer

StructureNodeKind is a type which is specific to a particular feature,
file_structure. It shouldn't be in the "code shared by all ide features"
part.

crates/ide/src/file_structure.rs

@@ -1,4 +1,4 @@
-use ide_db::{StructureNodeKind, SymbolKind};
+use ide_db::SymbolKind;
 use syntax::{
     ast::{self, AttrsOwner, GenericParamsOwner, NameOwner},
     match_ast, AstNode, AstToken, NodeOrToken, SourceFile, SyntaxNode, SyntaxToken, TextRange,
@@ -16,6 +16,12 @@ pub struct StructureNode {
     pub deprecated: bool,
 }
 
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, PartialOrd, Ord)]
+pub enum StructureNodeKind {
+    SymbolKind(SymbolKind),
+    Region,
+}
+
 // Feature: File Structure
 //
 // Provides a tree of the symbols defined in the file. Can be used to