Have a `buf source migrate` command

[doriable]

Feature

Similar to buf config migrate, which migrates/upgrades the buf.{yaml,gen.yaml} config files, it would be nice to have a command in buf source which is able to migrate/upgrade the syntax/edition of the proto source.

Example UX

buf source migrate <input> [--syntax, --diff]

• The input default is . if not specified, and is otherwise a source input (dir/on-disk modules/workspace or protofileref), same behaviour as other commands we have.
• The user can optionally specify a --syntax, e.g. --syntax=proto3 or --syntax=2024 (for editions 2024). If no syntax is specified, we'll use the latest implemented edition of the compiler the CLI version current depends on (https://github.com/bufbuild/protocompile/blob/aa1a63ac3dac8c2eb5bbebf21d6a8b5c0b8f8e5b/experimental/ast/syntax/editions.go#L24)
• If the source is already on the specified syntax and/or the source is already on the latest syntax, it's a no-op on the source file
• --diff produces only a diff output, if any, and does not write in-place. The default behaviour writes in-place. This is the same UX as buf config migrate (although I am open to deviating from that, just wanted to use an existing frame of reference for consistency)