WIP: Add client-side prediction for knife hits

[Rainyan]

Description

• Predict knife damage impact effects for better responsiveness for high latency players
• Fix issues with backstab damage not working despite being within the backstab angle
• Move backstab angle to a cvar, and convert from radians to degrees for more intuitive adjustment
• Add a cvar for toggling whether the pitch component of the view angle is considered in backstab calculations

Examples

300 ms roundtrip latency (net fakelag 150;) without prediction (current state):

knife-impacts-unpredicted.mp4

Same but with prediction (this PR):

knife-impacts-predicted.mp4

Note how the blood impact effect is not delayed in the latter, and renders from the correct position. The sound of the video embeds seems to be bugged, but it shouldn't be desynced in either case in reality.

Toolchain

• Windows MSVC VS2022

Linked Issues

• fixes Knife should be predicted #1904
• related Backstabs #404

[Rainyan]

The new bot_mimic_mirror 1 (together with bot_mimic 1) can be used to make the bots look the exact opposite direction (pitch included) for testing backstab mechanics when in close proximity. I didn't nerf the "stair stabbing" mechanic whereby one can land a backstab from the vertical axis, but there is a new cvar sv_neo_backstab_ignorez (default 1) which can be disabled to make it possible to "dodge" a vertical backstab my matching your look direction accordingly, analogous to how one can dodge the regular backstab by using the look yaw angle and continuously facing toward the opponent. Unsure which mode we eventually want, but I'll leave changing that gameplay stuff outside the scope of this PR.