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-lsp/src/references.rs

@@ -0,0 +1,62 @@
+use std::collections::HashMap;
+
+use aster_core::analysis;
+use aster_core::lexer::Lexer;
+use lsp_server::{Connection, Message, Request};
+use lsp_types::{Location, Position, Range, ReferenceParams, Uri};
+
+use crate::completion;
+
+pub fn handle_references(
+    documents: &HashMap<Uri, String>,
+    connection: &Connection,
+    req: Request,
+) {
+    let id = req.id;
+    let params: ReferenceParams = match serde_json::from_value(req.params) {
+        Ok(p) => p,
+        Err(e) => {
+            eprintln!("Invalid references params: {}", e);
+            return;
+        }
+    };
+
+    let uri = params.text_document_position.text_document.uri;
+    let position = params.text_document_position.position;
+
+    let text = documents.get(&uri).map(|s| s.as_str()).unwrap_or("");
+    let word = completion::get_word_at_position(text, position);
+
+    if word.is_empty() {
+        let resp = lsp_server::Response::new_ok::<Option<Vec<Location>>>(id, None);
+        let _ = connection.sender.send(Message::Response(resp));
+        return;
+    }
+
+    let (tokens, _) = Lexer::new(text).tokenize();
+    let positions = analysis::find_all_references(&tokens, &word);
+
+    let locations: Vec<Location> = positions
+        .into_iter()
+        .map(|(line, col)| {
+            let l = (line as u32).saturating_sub(1);
+            let c = (col as u32).saturating_sub(1);
+            Location {
+                uri: uri.clone(),
+                range: Range {
+                    start: Position {
+                        line: l,
+                        character: c,
+                    },
+                    end: Position {
+                        line: l,
+                        character: c + word.len() as u32,
+                    },
+                },
+            }
+        })
+        .collect();
+
+    let resp = lsp_server::Response::new_ok(id, locations);
+    let _ = connection.sender.send(Message::Response(resp));
+}