[Feature] Support "shell" parameter to `Executable.file`

[schwa]

Is your feature request related to a problem? Please describe.

It's quite common to need to run a command through the user's shell - in order to get access to whatever path they have set up in their various profile scripts.

Let's say the user has cowsay installed via homebrew (likely installed at /opt/homebrew/bin/ on Apple Silicon Macs) then:

try await run(.name("cowsay"))

Will fail because it can't find cowsay in the user's path.

Describe the solution you'd like

What I think is the best solution to this would to provide an API like that works like this:

try await run(.name("cowsay", shell: .user))

The shell parameter would instruct Subprocess to look up the user's shell via getpwuid and effectively convert the command into the following, selecting the user's shell as appropriate, and running it through the shell with -l as a login shell.

/opt/homebrew/bin/fish -l -c cowsay

Describe alternatives you've considered

It's possible for user code to do themselves or to provide UI for the user to specify their shell. But this feels something that Subprocess should provide.

Additional context
I am not sure how this should work on Windows.
Also the -l -c and/or other shell arguments impose some complexity on how the final arguments are constructed.

[schwa]

Another option is to add a useShell flag to the run function. We're able to do this ourselves in user code (but not we lose the advantage of using Arguments because user code has no ability to modify the Arguments value):

public func run<
    Input: InputProtocol,
    Output: OutputProtocol,
    Error: OutputProtocol
>(
    _ executable: Executable,
    useShell: Bool,
    arguments: [String] = [],
    environment: Subprocess.Environment = .inherit,
    workingDirectory: FilePath? = nil,
    platformOptions: PlatformOptions = PlatformOptions(),
    input: Input = .none,
    output: Output = .string,
    error: Error = .discarded
) async throws -> CollectedResult<Output, Error> {
    var executable = executable
    var arguments = arguments
    if useShell  {
        let shell: String
        if let pw = getpwuid(getuid()), let shellCString = pw.pointee.pw_shell {
            shell = String(cString: shellCString)
        }
        else {
            shell = "/bin/sh"
        }
        arguments = ["-l", "-c", try executable.resolveExecutablePath(in: environment).string] + arguments
        executable = .path(FilePath(shell))
    }
    let configuration = Configuration(
        executable: executable,
        arguments: Arguments(arguments),
        environment: environment,
        workingDirectory: workingDirectory,
        platformOptions: platformOptions
    )
    return try await run(
        configuration,
        input: input,
        output: output,
        error: error
    )
}