Adding a Gatsby example app

[hannah-smartbear]

Creating a Gatsby example app with BugSnag integrated. We can then direct any customers setting BugSnag up in their app to the example app, since we don't have official support for Gatsby. Similar to what we do with Next.js.

Created using Gatsby's quick start instructions (npm init Gatsby)

[github-actions[bot]]

@bugsnag/browser bundle size diff

	Minified	Minfied + Gzipped
Before	43.63 kB	13.37 kB
After	43.63 kB	13.37 kB
±	No change	No change

code coverage diff

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Generated by :no_entry_sign: dangerJS against 766a012ed2d851bc2315affefe3d30e08a50b860

[tomlongridge]

This has now been done in #2054