Add "type": "module" to package.json (#309)

It ensures that the ESM build of Vite is used instead of the CJS build
(deprecated). See https://vitejs.dev/guide/troubleshooting.html#vite-cjs-node-api-deprecated

This is also consistent with the Vite React templates:
https://github.com/vitejs/vite/blob/cdc664d4ea98d5373c3ceb2213771f1dfa4bb457/packages/create-vite/template-react-ts/package.json#L5

Because we were using TypeScript to begin with (and therefore the ESM
syntax for import/export), no change is required in source files.

Note that because ESM ("es") was already the default output format for
rollupjs, I don't think there is any difference in the final app
(index.html was already using `<script type="module">`). So in essence,
this change is really about avoiding the deprecation warning from Vite.

.eslintrc.js had to be renamed to .eslintrc.cjs. ESLint currently does
not support ESM configuration.

Signed-off-by: Antonin Bas <[email protected]>

client/web/antrea-ui/package.json

@@ -2,6 +2,7 @@
   "name": "antrea-ui",
   "version": "0.1.0",
   "private": true,
+  "type": "module",
   "dependencies": {
     "@cds/city": "^1.1.0",
     "@cds/core": "^6.5.0",