feat: IDE支持 — workspace拆分、LSP服务器、VS Code扩展

**Workspace 拆分**
- aster-core: 纯库，zero-dependency，包含 lexer/parser/ast/interpreter/error/analysis
- aster: REPL 二进制，薄封装 aster-core
- aster-lsp: LSP 语言服务器

**VS Code 扩展 (vscode-ext/)**
- TextMate 语法高亮 (.ast 文件)
- 语言配置 (注释切换、括号配对、自动缩进)
- LSP 客户端 (extension.js)

**LSP 服务端功能**
- Diagnostics: 实时显示 lex/parse 错误红色波浪线
- Completion: 关键字 + 内置函数 + 用户定义符号补全
- Hover: 悬停显示变量/函数信息
- Signature Help: 函数参数提示
- Goto Definition: Ctrl+Click 跳转到声明处
- Find References: 查找所有引用位置
- Rename: F2 重命名符号

**新增 analysis 模块**
- collect_symbols: AST 遍历收集符号
- find_declaration/find_all_references: Token 扫描定位声明和引用

aster-core/src/interpreter/env.rs

@@ -0,0 +1,50 @@
+use super::Value;
+use std::collections::HashMap;
+use std::rc::Rc;
+use std::cell::RefCell;
+
+#[derive(Debug)]
+pub struct Env {
+    /// (value, is_mutable) — `is_mutable` is true for `let`, false for `const`.
+    pub values: HashMap<String, (Value, bool)>,
+    pub parent: Option<Rc<RefCell<Env>>>,
+}
+
+impl Env {
+    pub fn new(parent: Option<Rc<RefCell<Env>>>) -> Self {
+        Self {
+            values: HashMap::new(),
+            parent,
+        }
+    }
+
+    pub fn define(&mut self, name: String, val: Value, mutable: bool) {
+        self.values.insert(name, (val, mutable));
+    }
+
+    /// Assign a new value to an existing binding. Returns `Err(msg)` if the
+    /// binding exists but was declared with `const` (immutable).
+    pub fn assign(&mut self, name: &str, val: Value) -> Result<bool, String> {
+        if let Some((_, false)) = self.values.get(name) {
+            return Err(format!("Cannot reassign constant '{}'", name));
+        }
+        if self.values.contains_key(name) {
+            self.values.insert(name.to_string(), (val, true));
+            Ok(true)
+        } else if let Some(parent) = &self.parent {
+            parent.borrow_mut().assign(name, val)
+        } else {
+            Ok(false)
+        }
+    }
+
+    pub fn get(&self, name: &str) -> Option<Value> {
+        if let Some((val, _)) = self.values.get(name) {
+            Some(val.clone())
+        } else if let Some(parent) = &self.parent {
+            parent.borrow().get(name)
+        } else {
+            None
+        }
+    }
+}