fix(flaws): ignore casing of text fragments

[caugner]

Summary

Fixes #8313.

Problem

Text fragments were reported as "broken links" if they contained a non-lowercase letter.

Solution

Exclude hashes from the check that start with the fragment directive (:~:).

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Screenshots

Before

After

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How did you test this change?

1. Locally reverted the changes of Run "yarn tool fix-flaws" on /web content#28009 to files/en-us/web/text_fragments/index.md, because it changed those fragment directives to lowercase.
2. Ran yarn && yarn dev with REACT_APP_WRITER_MODE=true and opened http://localhost:5042/en-US/docs/Web/Text_fragments#_flaws locally.

[LeoMcA]

lgtm