Add enableLegacyExtension flag to frontend config to make sg start web-standalone work again

Created by: philipp-spiess

We broke extensions when the dev server is started via sg start web-standalone since we don't properly initialize the config for this env (and default value for experimentalFeatures is an empty flag).

This needs to be updated when we change the default (I'll make an issue to clean up)

Test plan

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.