add maxFileSize

[gwyneplaine]

Add maxFileSize to keystone config, with a sensible default

[changeset-bot]

🦋 Changeset detected

Latest commit: 3eab9ab

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 12 packages

Name	Type
@keystone-next/keystone	Minor
@keystone-next/test-utils-legacy	Patch
@keystone-next/auth	Major
@keystone-next/example-auth	Patch
@keystone-next/app-basic	Patch
@keystone-next/example-ecommerce	Patch
keystone-next-app	Patch
@keystone-next/example-roles	Patch
@keystone-next/example-sandbox	Patch
@keystone-next/example-todo	Patch

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[timleslie]

👍 pending minor cosmetic changes in comments

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Sandbox	Source
@keystone-next/example-sandbox	Configuration