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