Limit the amount of horizontal splits you can do

[janhrastnik]

Hello,
this PR would address #1084 by setting a limit to the number of horizontal splits you can do.
If there exists a view with a height of 1 (only the status bar is drawn) after hsplit is run, then the newly created view is closed
and we refocus the previous view.

[the-mikedavis]

I think we will also need this check in split and hsplit_new

helix/helix-term/src/commands.rs

Lines 4242 to 4265 in a19eee6

	// split helper, clear it later
	fn split(cx: &mut Context, action: Action) {
	let (view, doc) = current!(cx.editor);
	let id = doc.id();
	let selection = doc.selection(view.id).clone();
	let offset = view.offset;
	cx.editor.switch(id, action);
	// match the selection in the previous view
	let (view, doc) = current!(cx.editor);
	doc.set_selection(view.id, selection);
	// match the view scroll offset (switch doesn't handle this fully
	// since the selection is only matched after the split)
	view.offset = offset;
	}
	fn hsplit(cx: &mut Context) {
	split(cx, Action::HorizontalSplit);
	}
	fn hsplit_new(cx: &mut Context) {
	cx.editor.new_file(Action::HorizontalSplit);
	}

(for repeated splitting by <C-w><C-s>)

[janhrastnik]

I didn't include the check in split in commands.rs since vsplit also calls that and I seperated the check into its own function. I hope that's all good.