ConVar flags that adapts to different Source 2 SDK versions

[Matlord93]

This section introduces a compatibility layer for setting ConVar flags that adapts to different Source 2 SDK versions.
Older and newer SDK builds expose slightly different APIs for modifying ConVar flags (AddFlags, ClearFlags, RemoveFlags, or SetFlag).
To ensure compatibility across these versions, the new implementation detects at compile time which API is available and uses it automatically.

The concept definitions check whether the ConVar type provides specific flag functions:

HasAddClear: uses AddFlags() and ClearFlags()

HasAddRemove: uses AddFlags() and RemoveFlags()

HasSetFlagBit: uses SetFlag(bit, bool)

The SetAllFlagsCompat function then compares the current flags with the desired ones and applies the correct operations:

Adds or clears bits using whichever API the SDK exposes.

Falls back to setting individual bits if only SetFlag() is available.

This change is required for the current Source 2 SDK,
as the internal ConVar implementation and flag management functions have changed between builds.
The new logic ensures consistent behavior without relying on private members such as m_nFlags,
making the code safer, forward-compatible, and SDK-agnostic.

💬 In short:

This update introduces a flexible flag-handling mechanism required by the latest Source 2 SDK to remain compatible with internal API changes and to avoid direct access to private ConVar data.

[Matlord93]

Fix issues #1013 and #1024

[Matlord93]

Please run the workflow again and check whether everything has been written and adjusted correctly.

I have now followed the guidelines and made this adjustment.

The change is currently compatible with the new update as well as the old Linux version