fix(button group): prevent unpress in single mode button group

[katebatura]

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Short description of what this resolves:

Closes #2773

In <nb-button-group> clicking 2 times on the same button don't remove the pressed state (similar to the radio group).

[yggg]

Lets add a comment clarifying the case when we need to prevent pressed state change. And what do you think about changing to:

Suggested change

	if (!this.buttonGroup?.multiple && this.pressed) {
	return;
	}
	this.pressed = !this.pressed;
	if (this.buttonGroup?.multiple || !this.pressed) {
	this.pressed = !this.pressed;
	}