fix a crash when parsing alert block syntax

devoncarew commented 7 months ago

fix a crash when parsing alert block syntax (fix #584)
have alert block syntax support multiple paragraphs (close #577)

@srawlins - this also revs the package version in preparation for publishing (so we can get the fix out for pub.dev).

[x] I’ve reviewed the contributor guide and applied the relevant portions to this PR.

Contribution guidelines:

- See our [contributor guide](https://github.com/dart-lang/.github/blob/main/CONTRIBUTING.md) for general expectations for PRs. - Larger or significant changes should be discussed in an issue before creating a PR. - Contributions to our repos should follow the [Dart style guide](https://dart.dev/guides/language/effective-dart) and use `dart format`. - Most changes should add an entry to the changelog and may need to [rev the pubspec package version](https://github.com/dart-lang/sdk/wiki/External-Package-Maintenance#making-a-change). - Changes to packages require [corresponding tests](https://github.com/dart-lang/.github/blob/main/CONTRIBUTING.md#Testing). Note that many Dart repos have a weekly cadence for reviewing PRs - please allow for some latency before initial review feedback.