feat: allow loading predefined migrations from proper exports

[AlessioGr]

Currently, predefined migrations can only be loaded if they are part of one of our db adapters.

With this PR, plugins will be able to export their own predefined migrations that can be created like this:

pnpm payload migrate:create --file @payloadcms/plugin-someplugin/someMigration

with the plugin exporting it in their package.json:

  "exports": {
    "./someMigration": {
      "import": "./someMigration.mjs",
      "types": "./someMigration.mjs",
      "default": "./someMigration.mjs"
    }
  },