Consider adding constants for NPC trade text pointers

[SatoMew]

Proposed by @Narishma-gb in #493 (comment)

Slightly off-topic, would you consider using constants in in_game_trades.asm ? Trade text dialogs are indexed at wInGameTradeTextPointerTableIndex, and are used as raw numbers in DoInGameTradeDialogue.

; in game trade text pointers
; TradeTextPointersX indexes (see engine/events/in_game_trades.asm)
	const_def
	const TEXT_WANNA_TRADE ; 0
	const TEXT_NO_TRADE    ; 1
	const TEXT_WRONG_MON   ; 2
	const TEXT_THANKS      ; 3
	const TEXT_AFTER_TRADE ; 4

I used these constant names in Pokegreen, see https://github.com/Narishma-gb/pokegreen/blob/24ad50fa9c7295e7b2fd003a7b40386cc02a7d32/engine/events/in_game_trades.asm