Add Objective-C language support (compiled_lang)

[darkn3rd]

Add Objective-C, using the GNUstep libobjc2 runtime (https://github.com/gnustep/libobjc2) rather than GCC's older bundled objc runtime, as a new language in lessons/compiled_lang/, alongside the existing C++/C#/Go/Java/Rust. See also https://askubuntu.com/questions/328343/how-to-compile-build-and-run-objective-c-program-in-ubuntu-using-terminal.

Based on cpp/ as the reference (same compile-then-link make model, via clang $(gnustep-config --objc-flags) $(gnustep-config --objc-libs) -lgnustep-base instead of a plain g++/gcc invocation).

This one has meaningfully more platform variance than the other compiled_lang additions - worth reading through before starting:

• macOS: fully native, no new packages needed - Apple's own Xcode CLI tools (already installed by macos_xcode_cli_tools) ship Objective-C support and libobjc.dylib out of the box.

• MSYS2: fully covered via pacman - mingw-w64-x86_64-libobjc2, mingw-w64-x86_64-gnustep-base, mingw-w64-x86_64-gnustep-make (and -gnustep-gui if a GUI-capable runtime is ever wanted) are all real, current packages.

• Ubuntu: apt install gobjc gets you gcc's own compiler frontend (cc1obj) and its older bundled objc runtime, but the modern libobjc2 runtime this issue is actually about (ARC, blocks, GCD) is confirmed not packaged for apt and needs building from source. Worth deciding whether the older gcc runtime is an acceptable substitute for the lesson content, or whether a from-source build step belongs in an ubuntu22_*-style custom script, matching how this project already handles non-packaged installs elsewhere.

• Windows: no Chocolatey package found, and no simple native-Windows install path exists - GNUstep's own Windows toolchain is either the MSYS2-based approach (i.e. not a separate native-Windows thing at all) or a much heavier MSVC-based toolchain (gnustep/tools-windows-msvc). Worth deciding whether Windows-native (choco:) Objective-C support is in scope at all, or whether this language is MSYS2-only on the Windows side.

• Cygwin: not confirmed either way in a first pass - needs checking directly against Cygwin's own package list.

• Resolve the open platform questions above (Ubuntu's gcc-vs-source-build tradeoff; Windows scope decision; confirm Cygwin) before or as part of implementation

• New lessons/compiled_lang/objc/ directory: Makefile(.win) (adapted from cpp/Makefile, using clang + gnustep-config flags instead of g++), Rakefile (imports shared testbox.rake), psakefile.ps1, src/ with one .m file per lesson (~60 files mirroring cpp/src's set), bin//target/ output dirs

• Add Objective-C's confirmed packages to lessons/compiled_lang/{Brewfile,choco.config,cygwin_pkgs.txt,msys2_pkgs.txt} per platform (macOS needs none beyond existing Xcode CLI tools; MSYS2 needs the three pacman packages above; Ubuntu/Windows pending the open questions)

• Add an Objective-C entry to the Compiled Languages area in scriptbox/scripts/verify_commands.rb's AREAS

• lessons/compiled_lang/README.md update

This is a full new-language port (~60 lesson files) with real platform-support open questions on top, so likely worth resolving the open questions and doing this in smaller PRs rather than one large one.

[darkn3rd]

Found a working apt-only path for the Ubuntu question above: https://github.com/palmercluff/Objective-C-Linux-Port

It installs build-essential gobjc gobjc++ gnustep gnustep-devel libgnustep-base-dev (all Debian apt packages, no source build), sources /usr/share/GNUstep/Makefiles/GNUstep.sh to set up the GNUstep build environment, then compiles directly. This is a different path than what I originally found - libgnustep-base-dev apparently pulls in an adequate objc runtime as a dependency even though it isn't literally named libobjc2, contradicting my earlier claim that the modern runtime "needs building from source" on apt.

Worth verifying directly against Ubuntu 22.04 specifically before relying on it (the guide targets Debian 8, an old release, so package availability/behavior should be re-confirmed on this project's actual target), but this looks like the real answer to the Ubuntu open question - no source build needed after all.