Add horizontal door component

[wjt]

Currently we only have a door asset & and component that blocks vertical movement, in scenes/game_elements/props/door:

Closed	Open

These assets are the same size: 64px wide (one tile) but 88px high.

We generally place it between two clifftops:

and use it to block player progression until some condition is reached.

When laying out levels, it is annoying that we can't place a door in front of a horizontal passage. We should add a similar-looking door (or if it can't be made to look good, a door-like component) to block horizontal movement.

XP Summary (total 475)

Subdomain	Skill	XP
Game Design: Core Gameplay & Mechanics	Problem Solving	50
Game Design: Core Gameplay & Mechanics	Creativity & Design Thinking	50
Game Design: Core Gameplay & Mechanics	Technical Literacy	75
Art: Aesthetics & Visual Design	Problem Solving	75
Art: Aesthetics & Visual Design	Creativity & Design Thinking	75
Art: Aesthetics & Visual Design	Technical Literacy	150

[wjt]

@PlayMatters You sent me these horizontal door assets:

These are not usable in-game as it stands because the background is solid white, not transparent.

[wjt]

I also think it's going to be visually tricky to make them work with clifftops. Here's an example of a horizontal gap in some cliffs (I just adjusted ink_combat_round_2 because it was open):

Here I've used the rounded "end-of-cliff" tiles above and below the gap. I think the door will look weird against these, so we'll probably need to use the square non-corner tiles to make the door join them cleanly:

[wjt]

but I think the door will need to be taller than 64px because it will want to both visibly block the left-hand or right-hand side of the 64x64 walkable grass tile, but also protrude upwards so that it looks like it's joined to the cliff.