Re-add support for nested deadkeys

[snomos]

According to @bbqsrc the structure in this section should work. But when tested with keyboard-fkv, it fails:

cd keyboard-fkv
./autogen.sh
./configure
make mac    

*** Building keyboards for the fkv language. ***

CDPATH="${ZSH_VERSION+.}:" && cd . && /bin/sh '/Users/smo036/langtech/gut/giellalt/keyboard-fkv/build-aux/missing' autoconf

*** macOS build! ***

  KBDGEN   mac
2024-01-05T12:02:53.358589Z  INFO kbdgen: kbdgen 3.0.0-beta.4
2024-01-05T12:02:53.359345Z  INFO kbdgen::bundle: Canonical Bundle Path: "/Users/smo036/langtech/gut/giellalt/keyboard-fkv/fkv.kbdgen"
thread 'main' panicked at 'not yet implemented: Recursion required ahead', src/build/macos/generate_macos.rs:391:41
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
make: *** [mac] Error 101

CI gives the same error message.

It seems that this is a feature of older kbdgen that has not yet been implemented in the Rust version. Commenting out the nested deadkeys makes the build go through.