fix($markdown): line highlighting not working correctly when importing code snippets

[diogotcorreia]

Summary

When importing code snippets with the <<< syntax, you could only either specify a single range ({34-37}) or a maximum of two lines to be highlighted ({3,6}).

This PR fixes the regex that was matching the syntax so it allows for unlimited line highlights like the ``` syntax.

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

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Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

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If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature
• Related documents have been updated
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To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

Before this PR, a snippet like

<<< @/path/to/file.js{5-7,10-16,25}

would output Invalid code snippet option.

Old regex:

New regex:

[StevenLikeWatermelon]

good job! it helped me a lot

[billyyyyy3320]

Thanks a lot!

Could you add a unit test for this change? (markdown/__tests__/snippet.spec.js)

[diogotcorreia]

@billyyyyy3320 Done! 👍

[billyyyyy3320]

LGTM