fix(gatsby): Use import for gatsby-browser.js instead of require

It seems that due to the `require` usage there, treeshaking is not working as expected. Just replacing this with using esm `import` seems to resolve this. I tried this in sentry-docs and a reproduction app and it seemed good to me.

Author: mydea

packages/gatsby/.eslintrc.js

@@ -15,6 +15,16 @@ module.exports = {
         project: ['../../tsconfig.dev.json'],
       },
     },
+    {
+      files: ['./gatsby-browser.js'],
+      env: {
+        browser: true,
+        node: false,
+      },
+      parserOptions: {
+        sourceType: 'module',
+      },
+    },
   ],
   extends: ['../../.eslintrc.js'],
 };

packages/gatsby/gatsby-browser.js

@@ -1,7 +1,7 @@
 /* eslint-disable no-console */
-const Sentry = require('@sentry/gatsby');
+import { init } from '@sentry/gatsby';
 
-exports.onClientEntry = function (_, pluginParams) {
+export function onClientEntry(_, pluginParams) {
   const isIntialized = isSentryInitialized();
   const areOptionsDefined = areSentryOptionsDefined(pluginParams);
 
@@ -24,12 +24,12 @@ exports.onClientEntry = function (_, pluginParams) {
     return;
   }
 
-  Sentry.init({
+  init({
     // eslint-disable-next-line no-undef
     dsn: __SENTRY_DSN__,
     ...pluginParams,
   });
-};
+}
 
 function isSentryInitialized() {
   // Although `window` should exist because we're in the browser (where this script