Build: Unbreak rollup watch mode

After updating from rollup 2.56.3 to 2.70.1, this broke because
upstream maps the extension-less variant to ESM. This will hopefully
be fixed soon, but is easy enough to workaround in the interim.

Workaround upstream rollup/rollup#4460.

build/watch.js

@@ -5,7 +5,7 @@ const path = require('path');
 const rollup = require('rollup');
 const kleur = require('kleur');
 const watch = require('node-watch');
-const loadConfigFile = require('rollup/dist/loadConfigFile');
+const loadConfigFile = require('rollup/dist/loadConfigFile.js');
 
 const { preprocess } = require('./dist-replace.js');