Let a plugin carry its own package.json and npm dependencies

plugin-api/index.ts

@@ -0,0 +1,2 @@
+// Re-export rather than the surface itself: a package's `exports` target may not escape its folder.
+export * from "../src/plugin-host/plugin-api.ts";

plugin-api/package.json

@@ -0,0 +1,7 @@
+{
+  "name": "@plainpages/plugin-api",
+  "version": "1.0.0",
+  "private": true,
+  "type": "module",
+  "exports": "./index.ts"
+}