Add the CODEGEN keyword to codegen-style add_custom_command() calls

[Randalphwa]

The PR's purpose — add the CODEGEN keyword to codegen-style add_custom_command() calls so consumers on CMake 3.31+ can build the builtin codegen target to generate resources without compiling them (ideal for clang-tidy CI) — was applied unchanged; only the diff granularity was adapted to the current 3.0.0 file structure:

1. Module top (line 40–50): if(POLICY CMP0171) / cmake_policy(SET CMP0171 NEW) — exactly the PR's guarded block. With our cmake_minimum_required(VERSION 3.12...4.0) ceiling, CMP0171 auto-activates NEW on CMake 3.31+ anyway, but the block covers consumers whose own project sets an older policy version. It's a no-op on CMake < 3.31, so the 3.12 minimum constraint is preserved.
2. cmrc_add_resource_library() (line 143–149): set(maybe_CODEGEN CODEGEN) / else() empty, appended as ${maybe_CODEGEN} to the resource-loader add_custom_command(OUTPUT "${libcpp}"...).
3. _cmrc_generate_intermediate_cpp() (line 285–290): same pattern, appended to the intermediate-file add_custom_command(OUTPUT "${outfile}"...).

Verification (local CMake 4.3.2, Ninja generator)

• Configured tests/ — no warnings/errors.
• --target codegen builds successfully.
• Proven behavior: in a fresh build_codegen scratch dir, codegen produced all 27 generated .cpp files (every resource loader + intermediate file) with zero .obj files and no project executables — generation without compilation, exactly the PR's goal.
• Full ctest --test-dir build --output-on-failure suite passes (all tests green), confirming no regression in the normal build path.

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Credit goes to @craigscott-crascit for the original idea -- I didn't use his commit exactly, since I had already changed CMakeRC, but the idea was valuable and worth doing.