Add C++Builder 13 support and fix packaging inconsistencies in C++Builder 12

[zencode1]

Summary

This adds C++Builder 13 packages (VirtualTreesCR/VirtualTreesCD) and, in the process of copying the C++Builder 12 packages as a starting point, fixes several ways the C++Builder 12 project folder had drifted from the conventions used by every other CBuilder version (10 through 11). Calling these out explicitly since they're fixes riding along with the new-version work, not just refactoring for its own sake:

• .groupproj was misnamed and had a mismatched GUID.** Renamed to VirtualTreeView.groupproj and corrected the GUID.
• VirtualTreesCD.cbproj / VirtualTreesCR.cbproj had mismatched GUIDs from other projects. Restored the shared GUIDs.
• ILINK_Description didn't match. All other versions use "VirtualTree CBuilder designtime/runtime package"; 12 had "VirtualTrees Designtime"/"VirtualTrees Runtime". Aligned the text.
• Missing DllSuffix. Set to "$(Auto)" to make future maintenance easier.
• BCC_UseClassicCompiler mismatched between Debug and Release. I set to false per Embarcadero's current recommendation.
• Both packages now explicitly declare DCC_OutputNeverBuildDcps and use "Explicit rebuild" build control, matching the convention of the other packages.

Platform support: Win32 only for the design-time package, for now

VirtualTreesCR (runtime) declares and builds cleanly for Win32, Win64, and Win64x. VirtualTreesCD (design-time) is intentionally limited to Win32 only on both 12 and 13. Embarcadero has added support in 13.2 so we can revisit this for future package changes.

Also included

• New Packages/CBuilder 13/ folder (VirtualTreesCR/VirtualTreesCD .cbproj/.cpp, VirtualTreeView.groupproj), built from the now-corrected CBuilder 12 packages with only the expected ProjectVersion bump (20.3 → 20.4).
• Demos/CBuilderMinimal: updated includes to unit-scoped VCL headers (System.Classes.hpp, Vcl.Controls.hpp, etc.) and added the #pragma link/#include lines added by the IDE due to the VirtualTrees.pas unit restructure.
• .gitignore: added entries for C++Builder files generated by modern tools.

Test plan

Built locally with RAD Studio 13.1 (Athens):

• Packages/CBuilder 12/VirtualTreeView.groupproj - Release/Win32 and Debug/Win32: 0 warnings, 0 errors (page-size linker warning is gone).
• Packages/CBuilder 12/VirtualTreeView.groupproj - VirtualTreesCR Release/Win64 and Release/Win64x: 0 warnings, 0 errors.
• Packages/CBuilder 13/VirtualTreeView.groupproj - Release/Win32 and Debug/Win32: 0 warnings, 0 errors.
• Packages/CBuilder 13/VirtualTreeView.groupproj - VirtualTreesCR Release/Win64x: 0 warnings, 0 errors.
• Demos/CBuilderMinimal/main.cpp and CBMinimal.cpp compile cleanly against the new includes/pragma links. Does not link due to a compiler mode mismatch. I decided not to change in this PR because it may deserve some discussion.

[Daniel-Tr]

Same as with the other PR regarding the C++ Builder: I reviewed the changed code, but I can't verify this in C++ Builder.