Add redo to Undoer

[YgorSouza]

Is your feature request related to a problem? Please describe.

The Undoer does not have a redo functionality, and thus neither does the TextEdit. most text boxes that have undo also have redo, and there is a TODO for it in the source code.

egui/crates/egui/src/widgets/text_edit/builder.rs

Lines 987 to 988 in 38b4234

	} if modifiers.command && !modifiers.shift => {
	// TODO(emilk): redo

Describe the solution you'd like

The Undoer should push undone states to a redo stack, and have a redo() method that can be called to revert the undo() call, as well as a has_redo() method to help set the enabled/disable state for the corresponding UI button.

Describe alternatives you've considered

It might be possible to check the state of has_undo() before and after calling feed_state() or add_undo(), and push a state to a redo stack stored separately, but that seems complicated and error-prone, and this feature is needed for the TextEdit anyway, so it may as well be provided by the Undoer.

Additional context

I am willing to try to implement this feature myself, but I would like to know if there are any pitfalls I should consider, that might have prevented it from being implemented until now.

[LoganDark]

any pitfalls I should consider, that might have prevented it from being implemented until now

I'm the author of the PR that just landed to close this. Seems like just laziness :)