Provide workspace_folders capability when initializing LSP

This fixes the YAML language server which was erroring on startup
after adding the `did_change_watched_files` capability in #2258.

crates/lsp/src/lsp.rs

@@ -335,6 +335,7 @@ impl LanguageServer {
                     did_change_configuration: Some(DynamicRegistrationClientCapabilities {
                         dynamic_registration: Some(true),
                     }),
+                    workspace_folders: Some(true),
                     ..Default::default()
                 }),
                 text_document: Some(TextDocumentClientCapabilities {