fix: enhancement of isExternal

[sy-records]

Summary

Fix an xss vulnerability that was fed via email.

Fix https://github.com/docsifyjs/docsify/issues/1477#issuecomment-1594666599

What kind of change does this PR introduce?

For any code change,

• [ ] Related documentation has been updated if needed
• [ ] Related tests have been updated or tests have been added

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

• [ ] Yes
• [x] No

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

Related issue, if any:

Tested in the following browsers:

• [x] Chrome
• [ ] Firefox
• [ ] Safari
• [ ] Edge
• [ ] IE

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Name	Status	Preview	Comments	Updated (UTC)
docsify-preview	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jun 17, 2023 0:23am

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[Koooooo-7]

Could u plz add a test case on this for good understanding of this changes ?

[Koooooo-7]

I checked the mail and yes that we need check the \ , is it harmful either when it contains more than 2 \\ ?

[sy-records]

One and more are problematic, so matching to one is OK.