Add language specification document

This commit introduces the BNF for the Myc language, along with its core
semantics, data types, and evaluation logic. It also details the macro
system and provides an overview of the standard library. This document
serves as a foundational context for LLMs to understand and generate Myc
code.

gemini.md

@@ -1,4 +1,4 @@
-## Rust-Portierung des Delphi-AST-Compilers
+# Rust-Portierung des Delphi-AST-Compilers
 
 * Dieses Repository enthält den Rust-Port des Delphi-Compilers.
 * Das Ziel ist, die alte Delphi-Codebase nach Rust zu portieren.
@@ -68,3 +68,9 @@ Options:
     --no-opt        Disable optimization
 -h, --help          Print help
 -V, --version       Print version
+
+# Sprachspezifikation
+
+* Dieses Dokument sollte aktuell gehalten werden.
+
+@docs/BNF.md