Add optional range parameter to SyntaxTreeParams

When range is provided, instead of showing the syntax for the whole file, we'll
show the syntax tree for the given range.

crates/ra_lsp_server/src/req.rs

@@ -39,6 +39,7 @@ impl Request for SyntaxTree {
 #[serde(rename_all = "camelCase")]
 pub struct SyntaxTreeParams {
     pub text_document: TextDocumentIdentifier,
+    pub range: Option<Range>,
 }
 
 pub enum ExtendSelection {}