fix(util): don't warn if a file contains only a Byte Order Mark (BOM)

[jamietanna]

Changes

As noted in #41381, it's common for a Byte Order Mark (BOM) Unicode
character to exist in a codebase that's used on Windows.

If we only find a BOM, we will report a TRACE log instead of the WARN,
but if any other characters are found, we'll continue with our WARN.

Context

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The public repository: https://github.com/Particular/Particular.Packaging