Support relative paths

[Bartleby2718]

Filing a separate issue (per https://github.com/madelson/MedallionShell/pull/111#discussion_r1615359632) to avoid a scope creep

[Bartleby2718]

Sample LINQPad code on Windows #116):

void Main()
{
    Command.Run(
        @"C:\Program Files\dotnet\dotnet.exe",
        new[] { "--version" })
        .GetOutputAndErrorLines()
        .Dump();
}

but the following fails:

void Main()
{
    Command.Run(
        @".\dotnet.exe",
        new[] { "--version" },
        o => o.WorkingDirectory(@"C:\Program Files\dotnet\"))
        .GetOutputAndErrorLines()
        .Dump();
}

[Bartleby2718]

@madelson This doesn't have to be an option, right?

[Bartleby2718]

^A pull request has been staged. Will file a PR in this repo if all tests pass and the Mono PR is merged.