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feat: expose the number of migrations to be applied

This is a slightly different take on #170, where instead of returning
the total number of migrations defined, we return the number of
migrations defined, but not yet applied.

Going back to the discussion in #170, what we actually need is:
1. tell whether migrations will be run, without actually running them.
   One use of this information is to run a backups before applying the
   migrations.
2. [“see the version difference somehow”][diff], i.e. the number of
   migrations to be applied.

Thus I think that returning the number of migrations to be applied,
instead of the number of migrations already applied fulfills the need
better. And it is less of a foot gun in my opinion:
1. The index in the vector of migration is off by one compared to the
   index in the user version, which is the one exposed by
   [`to_version()`][to_version] effectively. In other words, the first
   migration in the array is index 0, but that’s effectively version 1.
2. As discussed in #170, users might be tempted to write something like:
   ```rust
   if migrations.needs_to_run() {
     migrations.to_latest()
   }
   ```
   which would be suboptimal, as `to_latest` performs the same check.

Closes https://github.com/cljoly/rusqlite_migration/issues/170

[to_version]: https://docs.rs/rusqlite_migration/1.3.1/rusqlite_migration/struct.AsyncMigrations.html#example-4
[difference]: https://github.com/cljoly/rusqlite_migration/issues/170#issuecomment-2343979134

Builds-on: https://github.com/cljoly/rusqlite_migration/pull/171
Co-authored-by: Alex Helfet <alex.helfet@gmail.com>

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