add a tool that checks for a migration guide for a new major version

[chee]

few different ways to approach this:

• remark-lint preset for migrations that checks that every major version is documented in the migration guide (some will need to be added in the PR that adds this)
• standalone tool that checks a new migration guide section has been added if the PR contains a breaking commit

[chee]

this is slightly off-topic, but let's also write fixers for our remark-lint preset so the readme can be automatically fixed. we could also check that the README contains the correct table based on the MIGRATION.md, and generate the table with a fix rule