Dart pubspec updater doesn't work as expected for prerelease.

### Overview
In case of defining a version with prerelease version and without build number in pubspec.yaml, [dart pubspec updater](https://github.com/googleapis/release-please/blob/64ffba96734bf158818121c067870077d903611b/src/updaters/dart/pubspec-yaml.ts) updates the version with build number.

#### Expected

The version should be updated prerelease version and build number.

#### Actual

Dart pubspec updater recognizes prerelease version as build number.
https://github.com/googleapis/release-please/blob/64ffba96734bf158818121c067870077d903611b/src/updaters/dart/pubspec-yaml.ts#L29C39-L29C66

You can confirm this behavior from [this PR](https://github.com/ronnnnn/thema/pull/8/files).

#### Environment details

- googleapis/release-please-action@7987652d64b4581673a76e33ad5e98e3dd56832f

#### Steps to reproduce

1. Define version with prerelease version (like `version: 1.0.0-dev01`) on pubspec.yaml.
2. run googleapis/release-please-action on GitHub Actions.