Move PEG grammar into a dedicated source file

docs/README.md

@@ -9,7 +9,7 @@
 | 各个 crate 如何分层，哪些能力已实现 | [架构](architecture/README.md) | [crates](../crates/README.md) |
 | EndorObject、对象头、句柄、默认操作 | [对象模型](runtime/objects.md) | [core/src](../crates/core/src/README.md) |
 | Number、String、Dict、Cell、Function、异常数据 | [基本对象](runtime/builtins.md) | [core/src](../crates/core/src/README.md) |
-| 源码如何分词、PEG 规则、AST 和诊断 | [源码前端](runtime/frontend.md) | [frontend](../crates/frontend/README.md)、[CLI](../crates/cli/README.md) |
+| 源码如何分词、PEG 规则、AST 和诊断 | [源码前端](runtime/frontend.md) | [grammar.peg](../crates/frontend/src/grammar.peg)、[frontend](../crates/frontend/README.md)、[CLI](../crates/cli/README.md) |
 | 操作码、代码验证、局部槽、函数定义与调用协议 | [字节码 VM](runtime/bytecode.md) | instruction.rs / code.rs / frame.rs / eval.rs；见 [文件表](../crates/core/src/README.md) |
 | 基础信息保存在哪里、旧库如何迁移 | [宿主文件信息](operations/host-information.md) | [host](../crates/host/README.md) |
 | UTF-8 在日本地区是否适用，其他编码怎样进入 VM | [文本编码](runtime/text-encoding.md) | [core](../crates/core/README.md)、[host](../crates/host/README.md) |