Add a Platform class.

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

test/os/platform/is_posix.wren

@@ -0,0 +1,5 @@
+import "os" for Platform
+
+// Can't test for certain values since this test is cross-platform, but we can
+// at least make sure it is callable and returns a bool.
+System.print(Platform.isPosix is Bool) // expect: true

test/os/platform/name.wren

@@ -0,0 +1,5 @@
+import "os" for Platform
+
+// Can't test for certain values since this test is cross-platform, but we can
+// at least make sure it is callable and returns a string.
+System.print(Platform.name is String) // expect: true