[VScript] Add ability to send UserMessages

[horiuchii]

Description

This PR adds the CScriptUserMessage class which allows VScript to write data to an instance of this class and then call a member function to execute the message inside.
This would allow scripters to call User Messages only called by certain entities like game_text, game_text_tf, env_shake without the need to spawn them or messages that are currently impossible to call with VScript like ShowMenu, VGUIMenu and various others.

Usage

To execute a User Message in VScript, the user will call the global function UserMessageBegin() which takes a parameter for the message name.
After that, they can write any data they wish and then call the Execute() member function with a filter parameter.
This way, User Messages can be stored, modified and executed at any point in time.

Example Script

local UserMessage = UserMessageBegin("KeyHintText")
UserMessage.WriteByte(1)
local UserMessageInstance = UserMessage

UserMessage.WriteString("%+attack% Press this key.")
UserMessageInstance.WriteString("%+attack2% Or this key.")

UserMessage.Execute({
	filter_type = RECIPIENT_FILTER_SINGLE_PLAYER
	filter_entity = GetPlayerFromUserID(2)
})

UserMessage.Execute({
	filter_type = RECIPIENT_FILTER_SINGLE_PLAYER
	filter_entity = GetPlayerFromUserID(3)
})

UserMessageInstance.Execute({
	filter_type = RECIPIENT_FILTER_SINGLE_PLAYER
	filter_entity = GetPlayerFromUserID(4)
})