Initialize Aster project with basic structure, including Cargo configuration, lexer, parser, interpreter, and AST definitions. Add a sample script and README documentation. Implement basic error handling and environment management for variable storage.

src/ast/stmt.rs

@@ -0,0 +1,39 @@
+use crate::ast::expr::Expr;
+
+#[derive(Debug, Clone)]
+pub enum Stmt {
+    /// let x = expr;
+    Let {
+        name: String,
+        initializer: Expr,
+    },
+
+    /// 表达式语句：expr;
+    ExprStmt(Expr),
+
+    /// 代码块：{ ... }
+    Block(Vec<Stmt>),
+
+    /// if 语句
+    If {
+        condition: Expr,
+        then_branch: Box<Stmt>,
+        else_branch: Option<Box<Stmt>>,
+    },
+
+    /// while 循环
+    While {
+        condition: Expr,
+        body: Box<Stmt>,
+    },
+
+    /// 函数声明
+    Function {
+        name: String,
+        params: Vec<String>,
+        body: Vec<Stmt>,
+    },
+
+    /// return expr?;
+    Return(Option<Expr>),
+}