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/error/error.rs

@@ -0,0 +1,48 @@
+use crate::lexer::Token;
+
+#[derive(Debug)]
+pub enum RuntimeError {
+    LexError { message: String, line: usize, column: usize },
+    ParseError { message: String, token: Token },
+    RuntimeError { message: String, token: Option<Token> },
+}
+
+impl RuntimeError {
+    pub fn lex(message: impl Into<String>, line: usize, column: usize) -> Self {
+        RuntimeError::LexError {
+            message: message.into(),
+            line,
+            column,
+        }
+    }
+
+    pub fn parse(message: impl Into<String>, token: Token) -> Self {
+        RuntimeError::ParseError {
+            message: message.into(),
+            token,
+        }
+    }
+
+}
+
+impl std::fmt::Display for RuntimeError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            RuntimeError::LexError { message, line, column } => {
+                write!(f, "[Lex Error] {} at line {}, column {}", message, line, column)
+            }
+            RuntimeError::ParseError { message, token } => {
+                write!(f, "[Parse Error] {} at line {}, column {}", message, token.line, token.column)
+            }
+            RuntimeError::RuntimeError { message, token } => {
+                if let Some(tok) = token {
+                    write!(f, "[Runtime Error] {} at line {}, column {}", message, tok.line, tok.column)
+                } else {
+                    write!(f, "[Runtime Error] {}", message)
+                }
+            }
+        }
+    }
+}
+
+impl std::error::Error for RuntimeError {}