[3.x] Support adding custom items to editor right-click context menus

[citizenll]

Add right-click menu plugin functionality to implement this proposal

plugin.gd

tool
extends EditorPlugin
var FileSystemPlugin = preload("res://addons/csv-update-plugin/file_context_menu.gd")
func _enter_tree():
	filesystem_plugin = FileSystemPlugin.new()
	add_context_menu_plugin(EditorPlugin.CONTEXT_SLOT_FILESYSTEM, filesystem_plugin)

func _exit_tree():
	remove_context_menu_plugin(EditorPlugin.CONTEXT_SLOT_FILESYSTEM, filesystem_plugin)

file_context_menu.gd

extends EditorContextMenuPlugin
var icon = preload("res://addons/csv-update-plugin/context-icon.svg")
func _init():
	add_context_menu_item(icon, "Update csv", self, "handle_csv_update")


func can_handle(paths:Array):
	var has_cvs = false
	for path in paths:
		if path.get_extension()=="csv":
			has_cvs = true
	return has_cvs


func handle_csv_update(files):
	## files arg is Array<String>
	print("update csv files:",files)

In popMenu event handling, the parameter types for 3 different areas are different.

CONTEXT_SLOT_SCENE_TREE -> Array<Node>
CONTEXT_SLOT_FILESYSTEM -> Array<String>
CONTEXT_SLOT_SCRIPT_EDITOR -> Resource<GDScript>

Finally, I will explain the motivation behind this pull request. It mainly stems from my desire to create a plugin.
The general functionality is as follows: we use GoogleDocs as a place for game configuration (collaboration among multiple remote users).
In order to use this configuration in the game, I have developed a plugin that converts GoogleDocs to CSV (I also used another plugin of mine for configuration).
Each update from GoogleDocs involves updating the entire file. When the number of configuration files and data increases, our workflow slows down.
Therefore, I wondered if it would be possible to implement the operation of updating a single configuration file in the file browser area's right-click menu.
After searching around, I came across this proposal which had the same requirement, so I decided to attempt to implement it.
This is my first time contributing a pull request to the Godot Engine source code, and I'm not particularly skilled in C++. There may be areas in the code of this pull request that are not perfect. Thank you for pointing them out. 🤚~

[AThousandShips]

Same here

[AThousandShips]

This should be implemented for 4.x first and then considered for inclusion in 3.x, we do not normally accept features for 3.x exclusively unless they are not relevant for 4.x

[citizenll]

This should be implemented for 4.x first and then considered for inclusion in 3.x, we do not normally accept features for 3.x exclusively unless they are not relevant for 4.x

Currently I only work in 3.x, I will check this weekend if porting to 4.x is easy

[AThousandShips]

It might be some difficult changes involved, but it might not be too complicated either, but this might not get approved if it's just for 3.x depending

[KoBeWi]

This addresses #6128 (not sure if someone opened a new proposal).

In Godot 4 there is a Create New menu in FileSystem:

It would be nice to handle it as an option too. It would close godotengine/godot-proposals#8254

[citizenll]

This addresses #6128 (not sure if someone opened a new proposal).

In Godot 4 there is a Create New menu in FileSystem: It would be nice to handle it as an option too. It would close godotengine/godot-proposals#8254

The submenu option has been implemented in version 4.x, below is an example code.

extends EditorContextMenuPlugin
const PLUG = preload("res://addons/context-menu-plugin/plug.svg")

func _can_handle(paths: PackedStringArray) -> bool:
	return true

func _init() -> void:
	add_context_menu_item(PLUG, "Svg...", _handle_create_svg, SUBMENU_SLOT_FILESYSTEM_CREATE)

func _handle_create_svg(args):
	print(args)