[Shared] Add git tag to version string

[Razish]

replace ~160 lines of cmake with a single call to git 🤷 If the git tag can't be obtained (e.g. no git on path) it will default to UNKNOWN

So we have:

• version cvar (serverinfo) for the engine (also displayed in console)
• gamename cvar (serverinfo) for the module (also printed on module load)

Future work

Make the command execute at compile time, not just configuration time. Not an issue for official builds, but for local dev the commit hash will stick around until you reconfigure.