[flow][docs] Rename ESM website files to .mjs to silence Node warning

Summary:
Building the website printed `MODULE_TYPELESS_PACKAGE_JSON` because `website/src/js/flow-check-remark-plugin.js` (and the `initialized-flow-provider.js` it imports) use ES module syntax while sitting in a directory whose `package.json` has no `"type": "module"`. Node has to reparse the file as ESM after first parsing it as CommonJS, which triggers the warning.

The rest of `website/` (`docusaurus.config.js`, `babel.config.js`, `sidebars.js`, `src/js/flow-versions.js`) is CommonJS and is loaded via `require()`, so flipping the whole package to `"type": "module"` would be disruptive. Renaming just the two ESM files to `.mjs` is the targeted fix — Node treats `.mjs` as ESM unambiguously, no warning, no reparse. The dynamic `import()` in `docusaurus.config.js` is updated to point at the new path.

Changelog: [internal]

Reviewed By: marcoww6

Differential Revision: D105734893

fbshipit-source-id: 692b11296ebc9ecda13f7ad2ca529f690e045284

Author: gkz

website/docusaurus.config.js

@@ -247,7 +247,7 @@ module.exports = async () => ({
           }),
           remarkPlugins: [
             [require('@docusaurus/remark-plugin-npm2yarn'), {sync: true}],
-            (await import('./src/js/flow-check-remark-plugin.js')).default,
+            (await import('./src/js/flow-check-remark-plugin.mjs')).default,
           ],
         },
         staticDocsProject: 'flow',

…bsite/src/js/flow-check-remark-plugin.js …site/src/js/flow-check-remark-plugin.mjswebsite/src/js/flow-check-remark-plugin.js renamed to website/src/js/flow-check-remark-plugin.mjs website/src/js/flow-check-remark-plugin.js renamed to website/src/js/flow-check-remark-plugin.mjs

@@ -7,7 +7,7 @@
  * @format
  */
 
-import getFlowErrors from './initialized-flow-provider.js';
+import getFlowErrors from './initialized-flow-provider.mjs';
 
 /*:: type UnwrapPromise<T> = T extends Promise<infer V> ? V : T */