Windows editors often save UTF-8 files with a BOM (Byte Order Mark: \uFEFF).
When this is present, jsonc-parser reports InvalidSymbol at offset 0 because
the BOM is not valid JSON/JSONC syntax.
This commit strips the BOM before parsing, fixing issues #3164 where Windows
users report their opencode.jsonc file fails to parse even though it appears
to start with a valid '{' character.
Fixes: #3164
Co-authored-by: Jobdori <agent@yeongyu.kim>