Enable Remix future flags

[lizkenyon]

WHY are these changes introduced?

This PR enables all of the Remix future flags to opt into all of the changes coming in the next major version.

WHAT is this pull request doing?

Enable v3_fetcherPersist

• Changes
• Docs
• No code changes required

Enable v3_relativeSplatPath

• Changes
• Docs
• No code changes required, as we don't currently have a splat route with a relative link

Enable v3_throwAbortReason

• Changes
• Docs
• No code changes required

Enable v3_lazyRouteDiscovery

• Changes
• Docs
• No code changes required

Enable v3_singleFetch

• Changes
• Docs
• The json helper has been deprecated
• With this flag turned on though, we can instead return the raw data
• Update to use the new streamTimout API

Enable v3_routeConfig

• Changes
• Docs
• Adds two new packages to handle the new config base route configuration
• Adds the new app/routes.ts which is the new standard for defining routes.

Test this PR

shopify app init --template=https://github.com/Shopify/shopify-app-template-remix#<your-branch-name>

Checklist

• I have made changes to the README.md file and other related documentation, if applicable
• I have added an entry to CHANGELOG.md
• I'm aware I need to create a new release when this PR is merged

[matteodepalo]

LGTM. This is also what I recommended here.