Add a Platform class.

- Rename the "process" module to "os".
- Add Platform to it.
- Static "name" method.
- Static "isPosix" method.
- Docs and tests!

doc/site/modules/os/process.markdown

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+^title Process Class
+
+The Process class lets you work with operating system processes, including the
+currently running one.
+
+## Static Methods
+
+### **allArguments**
+
+The list of command-line arguments that were passed when the Wren process was
+spawned. This includes the Wren executable itself, the path to the file being
+run (if any), and any other options passed to Wren itself.
+
+If you run:
+
+    :::bash
+    $ wren file.wren arg
+
+This returns:
+
+    :::wren
+    System.print(Process.allArguments) //> ["wren", "file.wren", "arg"]
+
+### **arguments**
+
+The list of command-line arguments that were passed to your program when the
+Wren process was spawned. This does not include arguments handled by Wren
+itself.
+
+If you run:
+
+    :::bash
+    $ wren file.wren arg
+
+This returns:
+
+    :::wren
+    System.print(Process.arguments) //> ["arg"]