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/gatsby-browser.js

@@ -1,5 +1,5 @@
 /* eslint-disable no-console */
-const Sentry = require('@sentry/gatsby');
+import { init } from '@sentry/gatsby';
 
 exports.onClientEntry = function (_, pluginParams) {
   const isIntialized = isSentryInitialized();
@@ -24,7 +24,7 @@ exports.onClientEntry = function (_, pluginParams) {
     return;
   }
 
-  Sentry.init({
+  init({
     // eslint-disable-next-line no-undef
     dsn: __SENTRY_DSN__,
     ...pluginParams,