Initial working shell: bash-for-windows

- Cross-compiled Go-based shell for Windows (PE32+ executable)
- Builtins: cd, pwd, echo, exit, export, source, alias, type
- Coreutils: ls, cat, grep, sort, wc, head, find, cp, mv, rm, mkdir, touch, clear
- Command chaining: &&, ||, ;
- Pipes between builtins and external commands
- Variable expansion (, ) and assignment (NAME=VALUE)
- Tokenizer with single/double quote handling
- Linux and Windows (x86_64) builds via build.sh

.cargo/config.toml

@@ -0,0 +1,5 @@
+[target.x86_64-pc-windows-gnu]
+linker = "x86_64-w64-mingw32-gcc"
+
+[target.x86_64-pc-windows-gnullvm]
+linker = "x86_64-w64-mingw32-clang"