[sigma] Fixes bundle global name

This commit is related to #1450.
The global exported object is now properly named `Sigma`, instead of
`sigma`.

packages/sigma/vite.config.mts

@@ -3,7 +3,7 @@ import { defineConfig } from "vite";
 export default defineConfig({
   build: {
     lib: {
-      name: "sigma",
+      name: "Sigma",
       entry: "./src/index-bundle.ts",
       fileName: () => "sigma.min.js",
       formats: ["umd"],