darwin: return release version and os name

Return more details about macOS version, for example,
for current Big Sur it looks like:

  {"os":"darwin","release":"11.4","name":"macOS"}

Signed-off-by: Kyr Shatskyy <kyrylo.shatskyy@gmail.com>

index.js

@@ -19,10 +19,36 @@ module.exports = function getOs (cb) {
   var osName = os.platform()
   // Linux is a special case.
   if (osName === 'linux') return getLinuxDistro(cb)
+  if (osName === 'darwin') return getDarwinVersion(cb)
   // Else, node's builtin is acceptable.
   return cb(null, { os: osName })
 }
 
+function getDarwinVersion (cb) {
+  if (cachedDistro) return cb(null, cachedDistro)
+  var productVersionRegex = /^ProductVersion:\s+(.*)/m
+  var productNameRegex = /^ProductName:\s+(.*)/m
+  var exec = require('child_process').exec
+
+  distro = { os: 'darwin' }
+
+  exec('sw_vers', function (error, stdout, stderr) {
+      if (error) {
+          return cb(error, { os: os.platform() } )
+      }
+      var productVersion = stdout.match(productVersionRegex)
+      if (productVersion && productVersion.length === 2) {
+          distro.release = productVersion[1]
+      }
+      var name = stdout.match(productNameRegex)
+      if (name && name.length === 2) {
+          distro.name = name[1]
+      }
+      cachedDistro = distro
+      return cb(null, distro)
+   })
+}
+
 /**
  * Identify the actual distribution name on a linux box.
  */