fix: read updated shell shim paths

[martinrrm]

Summary

Update the POSIX shell-shim parser to recognize the path form emitted by current versions of cmd-shim.

cmd-shim@9.0.1 added support for invoking Windows executables from WSL. As part of that change, generated shebang-based shell shims began using:

"$basedir_win/<target>" "$@"

instead of:

"$basedir/<target>" "$@"

read-cmd-shim currently recognizes only the original $basedir form. As a result, it reports newer, valid shell shims as ENOTASHIM, even though they were generated by cmd-shim.

This change extends the existing parser to accept the optional, literal _win suffix. It continues to support older shims and returns the same relative target path for both forms.

Why this approach

The parser remains intentionally narrow:

• only $basedir and $basedir_win are recognized
• the existing quoting and "$@" structure are still required
• no shell expressions are evaluated
• .cmd and PowerShell parsing are unchanged
• asynchronous and synchronous readers continue to share the same extraction logic

This restores compatibility with current cmd-shim output without changing the public API or the result returned for existing shims.

Compatibility

This is backward-compatible:

• shims generated by older cmd-shim versions still use $basedir and remain supported
• shims generated by cmd-shim@9.0.1 and newer can now be read through $basedir_win
• consumers continue to receive the relative target path
• invalid or unrelated shell files continue to return ENOTASHIM

Testing

• all 18 integration tests pass
• both asynchronous and synchronous shebang shell-shim cases exercise the updated form through the current cmd-shim dependency
• 100% line, branch, and function coverage
• ESLint passes
• CI passes across Linux, macOS, and Windows on every configured Node.js version

Related change

The updated shell-shim form was introduced by npm/cmd-shim#178.