Massive restructure of plugin API

plug-api/silverbullet-syscall/markdown.ts

@@ -0,0 +1,7 @@
+import { syscall } from "./syscall.ts";
+
+import type { ParseTree } from "$sb/lib/tree.ts";
+
+export function parseMarkdown(text: string): Promise<ParseTree> {
+  return syscall("markdown.parseMarkdown", text);
+}