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

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[github-actions]

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%

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

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💛 - Coveralls

[srawlins]

These test cases look right to me; thanks!

[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).