fix: off-by-one bug in HTML whitespace removal

[rschristian]

Description

Fixes #14274 (comment)

startCol is 1-indexed, while startOffset is 0 indexed. This fix ensures magic-string isn't given a negative start position when checking a specific line.

Additional context

Same thing I mentioned on #14274 in that there doesn't seem to be any existing tests for the HTML output, just browser/integration tests, but I'd be happy to add a few if someone could direct me towards what's needed.

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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