Stripping utf8-bom before formatting files

[KaranPradhan266]

Fixes: #565

Strip UTF-8 BOM before formatting

Problem

Some Kotlin source files may begin with a UTF-8 Byte Order Mark (BOM, \uFEFF).
When such a file is passed to the formatter, the BOM is treated as part of the source text rather than metadata. This leads to an error while parsing.

Although BOMs are unnecessary for UTF-8 and discouraged for Kotlin source files, they can still appear due to editor defaults or generated files.

Solution

This change strips a leading UTF-8 BOM (\uFEFF) from file contents before formatting. By removing the BOM at input time:

• The formatter operates on clean Kotlin source text
• Behavior for files without a BOM is unchanged
• Formatting becomes robust against files saved with a UTF-8 BOM

This aligns with Kotlin tooling expectations and prevents BOM-related formatting errors without affecting valid source code.

Notes

• The BOM is removed only when it appears at the start of the file
• No formatting changes occur beyond this normalization step

[hick209]

Overall it LGTM.
Can you update the CHANGELOG.md before we merge this?

[KaranPradhan266]

@hick209 done!, Thanks for reviewing it!

[meta-codesync]

@hick209 has imported this pull request. If you are a Meta employee, you can view this in D89381568.

[meta-codesync]

@hick209 merged this pull request in d7b2226.