Provide a new file macro matching Node.js "process.platform" property

[lbssousa]

• Version: 20.2.0

• Target: Windows

I would like to suggest a new file macro ${platform} that would match the value of Node.js process.platform property (or change ${os} file macro to match it).

It would avoid some little annoyances in Electron entry file, like process.platform === 'win32' ? 'win' : (process.platform === 'darwin' ? 'mac' : process.platform) tests to match ${os} file macro with process.platform property.