bundling experiments (WIP)

Looking at ways to bundle custom widgets with Grist. WIP,
experimental, everything will need rewrite.

app/plugin/PluginManifest-ti.ts

@@ -14,6 +14,7 @@ export const BarePlugin = t.iface([], {
     "safeBrowser": t.opt("string"),
     "safePython": t.opt("string"),
     "unsafeNode": t.opt("string"),
+    "widgets": t.opt("string"),
     "deactivate": t.opt(t.iface([], {
       "inactivitySec": t.opt("number"),
     })),

app/plugin/PluginManifest.ts

@@ -82,6 +82,8 @@ export interface BarePlugin {
      */
     unsafeNode?: string;
 
+    widgets?: string;
+
     /**
      * Options for when to deactivate the plugin, i.e. when to stop any plugin processes. (Note
      * that we may in the future also add options for when to activate the plugin, which is for