os: ext => file_ext

vlib/os/os.v

@@ -617,7 +617,7 @@ fn print_c_errno() {
 	println('errno=$e err=$se')
 }
 
-pub fn ext(path string) string {
+pub fn file_ext(path string) string {
 	pos := path.last_index('.') or {
 		return ''
 	}

vlib/os/os_test.v

@@ -296,8 +296,8 @@ fn test_is_executable_writable_readable() {
 }
 
 fn test_ext() {
-	assert os.ext('file.v') == '.v'
-	assert os.ext('file') == ''
+	assert os.file_ext('file.v') == '.v'
+	assert os.file_ext('file') == ''
 }
 
 fn test_is_abs() {