Add showWorkspaceLoadedNotification to vscode client

This allows users to control whether or not they want to see the "workspace
loaded" notification.

This is done on the server side using InitializationOptions which are provided
by the client. By default show_workspace_loaded is true, meaning the
notification is sent.

crates/ra_lsp_server/src/init.rs

@@ -0,0 +1,39 @@
+use serde::{Deserialize, Deserializer};
+
+/// Client provided initialization options
+#[derive(Deserialize, Clone, Copy, Debug)]
+#[serde(rename_all = "camelCase")]
+pub struct InitializationOptions {
+    /// Whether the client supports our custom highlighting publishing decorations.
+    /// This is different to the highlightingOn setting, which is whether the user
+    /// wants our custom highlighting to be used.
+    ///
+    /// Defaults to `true`
+    #[serde(default = "bool_true", deserialize_with = "nullable_bool_true")]
+    pub publish_decorations: bool,
+
+    /// Whether or not the workspace loaded notification should be sent
+    ///
+    /// Defaults to `true`
+    #[serde(default = "bool_true", deserialize_with = "nullable_bool_true")]
+    pub show_workspace_loaded: bool,
+}
+
+impl Default for InitializationOptions {
+    fn default() -> InitializationOptions {
+        InitializationOptions { publish_decorations: true, show_workspace_loaded: true }
+    }
+}
+
+fn bool_true() -> bool {
+    true
+}
+
+/// Deserializes a null value to a bool true by default
+fn nullable_bool_true<'de, D>(deserializer: D) -> Result<bool, D::Error>
+where
+    D: Deserializer<'de>,
+{
+    let opt = Option::deserialize(deserializer)?;
+    Ok(opt.unwrap_or(true))
+}