Move babel.config.json to .babelrc

There are two other .babelrc in the project. I think it gives more
consistency and allows for comments.

frontend-shared/babel.config.json → frontend-shared/.babelrc.cjs

-{
+'use strict';
+
+module.exports = {
   "presets": [
     [
       "@babel/preset-react",
@@ -28,6 +30,8 @@
           {
             "development": true,
             "runtime": "automatic",
+            // Use `preact/compat/jsx-dev-runtime` which is an alias for `preact/jsx-runtime`.
+            // See https://github.com/preactjs/preact/issues/2974.
             "importSource": "preact/compat"
           }
         ]

scripts/gulp/frontend-shared.js

 'use strict';
 
-const fs = require('fs');
-
 const gulp = require('gulp');
 const babel = require('gulp-babel');
 const sourcemaps = require('gulp-sourcemaps');
@@ -10,10 +8,8 @@ const { run } = require('./run');
 
 const buildFrontendSharedJs = () => {
   // There does not appear to be a simple way of forcing gulp-babel to use a config
-  // file. Load it up as JSON and pass it in manually.
-  const babelConfig = JSON.parse(
-    fs.readFileSync('./frontend-shared/babel.config.json')
-  );
+  // file. Load it up and pass it in manually.
+  const babelConfig = require('../../frontend-shared/.babelrc.cjs');
 
   return (
     gulp

src/karma.config.js

@@ -102,7 +102,7 @@ module.exports = function (config) {
             // The existence of this preset option is due to a config issue with frontend-shared/
             // where jsx modules are not transpiled to js.
             // See https://github.com/hypothesis/client/issues/2929
-            presets: require('../frontend-shared/babel.config.json').presets,
+            presets: require('../frontend-shared/.babelrc.cjs').presets,
             extensions: ['.js'],
             plugins: [
               'mockable-imports',