Fixes #539 linter support, now checking YAML formats

plug-api/app_event.ts

@@ -7,6 +7,7 @@ export type AppEvent =
   | "editor:complete"
   | "minieditor:complete"
   | "slash:complete"
+  | "editor:lint"
   | "page:load"
   | "editor:init"
   | "editor:pageLoaded" // args: pageName, previousPage, isSynced

plug-api/types.ts

@@ -133,3 +133,10 @@ export type CodeWidgetContent = {
   markdown?: string;
   script?: string;
 };
+
+export type LintDiagnostic = {
+  from: number;
+  to: number;
+  severity: "error" | "warning" | "info" | "hint";
+  message: string;
+};