address a termination issue with GitHub alert syntax parsing

[devoncarew]

• address a termination issue with GitHub alert syntax parsing

When trying out the new alert syntax, it looks like the parsing never terminates - all the content after an alert is included in the alert.

cc @AlexV525

---

• [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.

[github-actions[bot]]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:markdown	7.2.1	ready to publish	v7.2.1

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.

[coveralls]

Pull Request Test Coverage Report for Build 7587761119

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.3%) to 96.135%

---

Totals
Change from base Build 7551994459:	0.3%
Covered Lines:	1542
Relevant Lines:	1604

---

💛 - Coveralls

[AlexV525]

@devoncarew Thanks for the fix!

[devoncarew]

For sure! I think there's one remaining - though much more minor - issue. You can't currently have alerts w/ multiple paragraphs (filed as #577).