Support configuration files with UTF-8 BOM (#20127)

changelog_unreleased/api/20127.md

@@ -0,0 +1,19 @@
+#### Support configuration files with UTF-8 BOM (#20127 by @fisker)
+
+<!-- prettier-ignore -->
+```console
+# Input
+$ cat .prettierrc.json
+{"semi":false}
+$ prettier example.js --check
+
+# Prettier stable
+Invalid configuration for file "example.js":
+JSON Error in .prettierrc.json:
+Unexpected token "" (\u{feff})
+
+# Prettier main
+Checking formatting...
+[warn] example.js
+[warn] Code style issues found in the above file. Run Prettier with --write to fix.
+```