You can make set multi-lined text in a button from within the startup function startupFcn().
The first line below creates two lines of text within the button.
The second line moves the button down and doubles its vertical size. Set the button width and the position of the top of the button from the DesignView.
function startupFcn(app, arg1)
app.Button.Text = ['Really long',newline,'button name'];
app.Button.Position([2,4]) = app.Button.Position([2,4]) + [-1,1].*app.Button.Position(4);
As of R2020b multi-line text in buttons can be achieved from Design View (thanks Eric Sargent).