port ujsx from Deno-only to cross-platform (Node/Bun/Deno)

Add npm project configuration (package.json, tsconfig.json, tsup, vitest)
matching the taskgraph_ts conventions. All source imports changed from .ts
to .js extensions for Node16 module resolution. Tests migrated from Deno.test
to vitest. Fixed strict type errors (noUncheckedIndexedAccess). Preserved
deno.json with sloppy-imports for dual Deno/Node compatibility.

Subpath exports: schema, h, reactive, context, events, pointer, host,
transform, jsx-runtime — plus barrel export at root.

Build: ESM + CJS dual output via tsup. 22 tests passing.

tsup.config.ts

@@ -0,0 +1,21 @@
+import { defineConfig } from 'tsup';
+
+export default defineConfig({
+  entry: {
+    mod: 'src/mod.ts',
+    'core/schema': 'src/core/schema.ts',
+    'core/h': 'src/core/h.ts',
+    'core/reactive': 'src/core/reactive.ts',
+    'core/context': 'src/core/context.ts',
+    'core/events': 'src/core/events.ts',
+    'core/pointer': 'src/core/pointer.ts',
+    'core/jsx-runtime': 'src/core/jsx-runtime.ts',
+    'host/config': 'src/host/config.ts',
+    'transform/registry': 'src/transform/registry.ts',
+  },
+  format: ['esm', 'cjs'],
+  dts: true,
+  sourcemap: true,
+  clean: true,
+  target: 'es2022',
+});