[enhancement] LoadMigrations with a given path

I am trying to use the package as a library.

For that, I created embedded FS and wanted to use LoadMigrations.

However, in `github.com/jackc/tern/v2 v2.3.2` that function expects to find migrations in the current "." folder whereas my FS requires a path.

```
//go:embed services/content/db/migrations/*.sql
var content embed.FS
```

Would you consider adding another Load function that takes a path parameter?

Start by reading LoadMigrations and the migration-loading callers to understand how the current-directory assumption is applied. Check how an embedded FS path would be passed, then define the additional loading entry point so migrations can be read from a supplied path. Done means callers can load the migrations shown in the issue from services/content/db/migrations without relying on the current directory.

Written by the indexing model from the issue text.