add hello_world demo

.gitignore

@@ -0,0 +1,8 @@
+# MSVC Windows builds of cjc generate these, which store debugging information
+*.cjo
+*.exe
+*.bchir2
+target/
+cjpm.lock
+.vscode/
+.cache/

samples/hello_world/main.cj

@@ -0,0 +1,5 @@
+// main函数最先执行
+main() {
+    // 打印Hello World!
+    println("Hello World!");
+}