Add CMake support for cross-platform builds

[toprakmurat]

Problem

The current build system uses a plain Makefile, which works well on Unix-like environments but is not portable to Windows, and lacks modern developer tooling support. Many users/devs expect CMake for cross-platform builds and IDE integration.

Proposed solution

• Add a CMakeLists.txt to provide CMake support for building librdesc, examples, and tests.
• Preserve FEATURES toggling (such as stack, dump_bnf, dump_dot) using CMake options or variables.
• Document CMake build instructions in the README.
• Ensure Linux/macOS/Windows/Mingw compatibility.

Motivation

CMake is the standard for portable C projects and simplifies integration with IDEs (Visual Studio, CLion, etc.), CI pipelines, and third-party package managers.