Add terrain generation plugin

[vldvoiculescu]

Terrain Generator is a procedural BSP brush generation plugin for NetRadiant-custom. It generates playable geometry (terrain and tunnels) as func_group entities directly inside the editor, using configurable noise-based heightmaps.

Supported shapes: Flat, Hill, Crater, Ridge, Slope, Volcano, Valley, Tunnel, Slope Tunnel

Key features:

• Generate from a selected brush (uses its XYZ bounds as the generation area) or from manual size input
• Adjustable sub-square size, noise type (Perlin/Simplex/Random), variance, frequency, and terrace stepping
• Texture selection integrated with Radiant's texture browser

Build command:

make MAKEFILE_CONF=msys2-Makefile.conf BUILD=debug DEPENDENCIES_CHECK=off DOWNLOAD_GAMEPACKS=no INSTALL_DLLS=no install/plugins/terrain_generator.dll