add @sys.exit (#74)

sys/internal/ffi/ffi.mbti

@@ -1,6 +1,8 @@
 package moonbitlang/x/sys/internal/ffi
 
 // Values
+fn exit(Int) -> Unit
+
 fn get_cli_args() -> Array[String]
 
 fn get_env_var() -> Map[String, String]

sys/internal/ffi/sys_js.mbt

@@ -40,3 +40,8 @@ extern "js" fn get_env_var_internal() -> Array[String] =
   #|   }
   #|   return result;
   #| }
+
+pub extern "js" fn exit(code : Int) -> Unit =
+  #| function(code) {
+  #|   process.exit(code);
+  #| }

sys/internal/ffi/sys_native.mbt

@@ -21,3 +21,5 @@ pub fn get_env_var() -> Map[String, String] {
   // not implement yet
   panic()
 }
+
+pub extern "native" fn exit(code : Int) -> Unit = "exit"

sys/internal/ffi/sys_wasm.mbt

@@ -32,3 +32,5 @@ pub fn get_env_var() -> Map[String, String] {
 }
 
 fn get_env_var_ffi() -> XExternStringArray = "__moonbit_fs_unstable" "env_get_vars"
+
+pub fn exit(code : Int) = "__moonbit_sys_unstable" "exit"

sys/sys.mbt

@@ -19,3 +19,7 @@ pub fn get_cli_args() -> Array[String] {
 pub fn get_env_var() -> Map[String, String] {
   @ffi.get_env_var()
 }
+
+pub fn exit(code : Int) -> Unit {
+  @ffi.exit(code)
+}

sys/sys.mbti

@@ -1,6 +1,8 @@
 package moonbitlang/x/sys
 
 // Values
+fn exit(Int) -> Unit
+
 fn get_cli_args() -> Array[String]
 
 fn get_env_var() -> Map[String, String]