[WIP/PoC] Automatic migrations

[zth]

Supersedes #7693

Left TODO for this first iteration

• Support migrating in .resi files as well
• Error on invalid config etc
• Actual command for running the migration
• Write a spec for the current state
• Add migrations for the remaining things in the JS namespace
• Add migrations for the Stdlib functions that have been deprecated
• (?) Publish a new Core version with migrations for only the APIs that have changed since merging into the compiler
• Consider if we want to deprecate the functions in Belt that are 1:1/fully covered in the new stdlib. If so, add deprecations and migrations for them as well

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commit: d8cc568