feat: rename process module to os and add Platform class with name and isPosix methods

Rename the "process" module to "os" across all source files, documentation, tests, and build configuration. Add a new Platform class to the os module with static methods `name` (returns OS string like "Windows", "Linux", "OS X", "iOS", "Unix", "POSIX", or "Unknown") and `isPosix` (returns bool indicating POSIX compliance). Update all import paths from "process" to "os" in test files and module registrations. Move process.c/h to os.c/h, rename processSetArguments to osSetArguments, and regenerate the os.wren.inc bytecode source. Add documentation pages for the os module and Platform class.

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