Add support for binding callback function pipelines

[MelanTech]

Here's a prototype implementation of callback function binding, which allows for functions to be triggered after data updates. Users can bind multiple custom callback functions using the with statement. This functionality is particularly useful for data storage, as users can save data via callback functions once it has been updated. Below is an example of its usage:

var _data = BindingSource.new(Data.new())
var _config_file = ConfigFile.new()

func _ready():
	_data.bind(&"count").using(_get_label).with(_on_data_changed).with(_on_data_changed_print)

func _on_button_pressed():
	_data.count += 1

func _on_data_changed(name, value):
	_config_file.set_value("section_name", name, value)
	_config_file.save("user://settings.cfg")

func _on_data_changed_print(name, value):
	prints(name, value)

class Data:
	var count: int = 0

[HotariTobu]

[SHOULD]
Change with_pipeline to callback_pipeline.

[SHOULD]
From the perspective of a with user, it's not clear from the code whether a callback function registered with with is called when the source changes or when the target changes.
Make it possible to specify whether the callback function is called on source changes or target changes when it is registered.
For example:

func with(callback: Callback, on_source_changed: bool = false):

[MUST]
Write tests.

[MUST]
Change the branch name to feat/MelanTech-callback-pipeline.
You'll need to recreate the PR to change the merge source branch name. Please finish your revisions and then create a new pull request.

[HotariTobu]

[SHOULD]
Do not pass null as a default value, just as you would with other arguments.

Suggested change

	with_pipeline: BindingWithPipeline = null,
	with_pipeline: BindingWithPipeline,

[HotariTobu]

[SHOULD]
Since source_property is known when the callback function is registered, passing it during the callback is redundant.
Instead, pass the value before the change.

Suggested change

	with_func.call(_source_property, next_source_value)
	with_func.call(source_value, next_source_value)

[HotariTobu]

Thank you for contributing to this add-on.
I have a few corrections I'd like you to make.