[TF2] Allow players to disable weapon shell ejection

[NachosChipeados]

Lets players disable weapon shell ejection by using the already existing cl_ejectbrass convar, which does exactly this in other source games such as HL2. Closes https://github.com/ValveSoftware/Source-1-Games/issues/5949

https://github.com/user-attachments/assets/8843c89c-449a-441f-a6a7-ca645cd485ef

Edit: This convar should also be added to user_default.scr in category 6, below tf_particles_disable_weather:

"cl_ejectbrass"
{
	"#Valve_EnableBrassEject"
	"#Tooltip_EnableBrassEject"
	{ BOOL }
	{ "1" }
}