parser: disallow statements after a return, branch check $if, $else, switch, and loops correctly

vlib/os/os.v

@@ -870,6 +870,7 @@ pub fn fork() int {
 		pid := C.fork()
 		return pid
 	}
+	panic('os.fork not supported in windows') // TODO
 }
 
 pub fn wait() int {
@@ -877,6 +878,7 @@ pub fn wait() int {
 		pid := C.wait(0)
 		return pid
 	}
+	panic('os.wait not supported in windows') // TODO
 }
 
 pub fn file_last_mod_unix(path string) int {

vlib/os/os_test.v

@@ -27,7 +27,6 @@ fn test_write_and_read_string_to_file() {
   
   read_hello := os.read_file(filename) or {
     panic('error reading file $filename')
-    return
   }
   assert hello == read_hello
 

vlib/os/os_win.v

@@ -40,6 +40,7 @@ pub fn get_module_filename(handle HANDLE) ?string {
             return error('Cannot get file name from handle.')
         }
     }
+    panic('this should be unreachable') // TODO remove unreachable after loop
 }
 
 // Ref - https://docs.microsoft.com/en-us/windows/win32/api/winbase/nf-winbase-formatmessagea#parameters