feat: rewrite as sh for POSIX and PowerShell for Windows

[coolaj86]

This was written years before I had a firm grasp on the POSIX shell, and before the ability to modify the PATH in PowerShell was widespread (the previous methods were buggy due to the 255 character limitation).

Go was a great choice at the time, and with my knowledge - but it isn't anymore.

The POSIX rewrite would include Webi's webi_path_add function and would be similar in nature to https://github.com/bnnanet/serviceman

Here's the secret for how to set the PATH on Windows in this new age:

[Environment]::SetEnvironmentVariable(
   "Path",
   [Environment]::GetEnvironmentVariable("Path", "User") + ";C:\your-path\zig-windows-x86_64-your-version",
   "User"
)

See also

• remove pathman in favor of posix & powershell functions webinstall/webi-installers#505
• https://github.com/webinstall/webi-installers/pull/612/files?diff=unified&w=1
• [Idea] Update PATH on Windows natively webinstall/webi-installers#428
• ref!: replace pathman with PowerShell [Environment]::SetEnvironmentVariable webinstall/webi-installers#599