Rename Position* nodes to Marker*

[Mickeon]

Closes godotengine/godot-proposals#5199.

As brought up in #54161 (comment).

The naming of this Node is odd, as it shares its name with one of its properties. Thus, assuming its name is unchanged, when fetching the position, it's possible to be in this situation:

@onready var position_2d := $Position2D
...
position_2d.position += Vector2.ONE # awkward to read

Thus, the comment above proposes an alternative name that I personally really like:

• Position2D -> Marker2D
• Position3D -> Marker3D

I believe it suits them a lot more.

Also changes their respective file names.

This renaming is not as obvious as they may be. If there's need for a proposal I'll make one.

[KoBeWi]

You also need to do add_compatibility_class().

[Riteo]

tbf I don't agree a lot with the choice of keeping the file names the same. If someone really cares looking for their history IIRC they should be able to see the first commit for these new files and notice the rename, then look for their previous history.

[Mickeon]

Oh, no, don't worry, I'll just change them in a following PR. It's to keep it one thing at a time. The reception to this PR is overwhelmingly positive, so I may do it, soon, as well.

[Riteo]

@Mickeon sorry, I don't get it, why should you do it in a separate PR?

[Mickeon]

I may be incredibly dumb, but you can look at this PR. I was able to tell Git that Position2D.xml and Position3D.xml have merely changed name, but the file is actually the same, and it recognised that. Because position_2d and position_3d have been modified internally, changing their file name would result in them being recognised as two wholy different files.

If there's a way to forcefully tell Git it's the same file, I don't know it.

Edit: I believe I still have to modify this PR, actually. My plan to keep history would not work, as there's position_2d comments around the place that would need to be changed, as well.

[Riteo]

@Mickeon I don't think that's that big of a deal? The commit is still very clear and from the diff one should still be able to see the old file name with the corresponding history.

I think that the renaming thing is just some optimization done by git and it has no other special property.

[Mickeon]

Got to have my cake and eat it, too. Updated the PR to change the file names, as well.

[akien-mga]

Seems fine to me. @reduz also appeared fine with that change.
And the PR and proposal are significantly upvoted, so this should be good to go.

[akien-mga]

Thanks!