Debugger template: allow missing or empty completion list (#10332)

It can be convenient to define project specific debugger templates, some of
which might not necessitate prompting the user to define completion.

This commit makes completion optional for debugger templates and starts the
dap immediately if undefined or empty.

helix-core/src/syntax.rs

@@ -510,6 +510,7 @@ pub enum DebugArgumentValue {
 pub struct DebugTemplate {
     pub name: String,
     pub request: String,
+    #[serde(default)]
     pub completion: Vec<DebugConfigCompletion>,
     pub args: HashMap<String, DebugArgumentValue>,
 }