Add args-file support for large Windows workspaces

[psandana]

Fixes #6934.

This implements the explicit argument-file option suggested in #6934 (comment) and replaces the closed #7027 approach.

Problem

cargo-fmt passes every target path for an edition directly to one rustfmt process. Windows limits the complete CreateProcessW command line to 32,767 UTF-16 code units, so large workspaces fail with OS error 206 before rustfmt starts.

Solution

Rustfmt gains an explicit argument-file option:

rustfmt --args-file <path>
rustfmt --args-file=<path>

Argument files are UTF-8 with one argument per line. They may be nested up to 16 levels, and -- terminates option and argument-file expansion globally. Ordinary filenames beginning with @ retain their existing meaning; no existing positional syntax is reinterpreted.

On Windows, cargo-fmt calculates the command-line length using Rust's Windows quoting rules. Only when an invocation would reach 32,767 UTF-16 code units does it write the complete rustfmt argument vector to a temporary file and invoke rustfmt --args-file <temporary-path>.

Keeping one rustfmt process per edition preserves existing session, ordering, JSON, and checkstyle behavior. User-provided --args-file options are expanded before cargo-fmt creates its transport file, so behavior remains identical below and above the Windows threshold.

Compatibility

• @source.rs remains an ordinary source filename.
• Non-Windows behavior is unchanged.
• Windows commands below the OS limit continue to use direct arguments.
• Argument-file paths and contents must be valid UTF-8, consistent with rustfmt's existing Unicode CLI.
• Embedded newlines are rejected because the format is one argument per line.
• The temporary file remains alive until rustfmt exits.

Tests

• Separated and = syntax, CRLF, blank arguments, missing files, nesting, and global -- termination.
• An ordinary source filename beginning with @.
• Windows command-line quoting and exact length calculation.
• A generated 500-target Windows workspace exceeding the limit.
• User-provided argument files across the cargo-fmt threshold.
• The original cargo fmt failure on Windows, too many command line args #6934 918-target reproducer.

[riverar]

May want to verify a UTF-8 args file starting with a byte-order mark (EF BB BF) is readable too.

[psandana]

Added UTF-8 BOM coverage in b7922a7. Argument-file readers now strip a leading EF BB BF, with unit coverage for both rustfmt and cargo-fmt plus BOM-enabled integration tests.