Respect --skip-source-first-line when formatting stdin - #5580

Description

--skip-source-first-line (-x) is honoured when formatting files by path, but it is silently ignored when the code is passed through stdin (black -x -). Instead, the first line is formatted along with the rest. When that line isn't valid Python, which is the case the option exists for (e.g. a Windows batch wrapper), Black fails to parse it:

$ printf '@echo off && python -x "%%~f0" %%* && goto :eof\nx=1\n' | black -x -
error: cannot parse: -:1:6

The same file formatted by path works. With --diff/--check on stdin, an unformatted first line also shows up as a change and makes the check fail.

format_file_in_place() splits the header off before decoding and prepends it again afterwards, but format_stdin_to_stdout() never looked at mode.skip_source_first_line. It now does the same. This includes the existing CRLF handling for headers, so the output doesn't end up with \r\r\n. The header is kept for write-back and for --diff/--color-diff output.

Checklist - did you ...

• Implement any code style changes under the --preview style, following the stability policy? (no style change, CLI/IO fix only)
• Add an entry in CHANGES.md if necessary? (under "Output")
• Add / update tests if necessary? (test_skip_source_first_line_via_stdin, LF and CRLF, write-back and --diff; fails without the fix)
• Add new / update outdated documentation? (no documentation change needed)