Set the default font to FONT.
When called interactively, prompt for the name of a font, and use
that font on the selected frame. When called from Lisp, FONT
should be a font name (a string), a font object, font entity, or
font spec.
If KEEP-SIZE is nil, keep the number of frame lines and columns
fixed. If KEEP-SIZE is non-nil (or with a prefix argument), try
to keep the current frame size fixed (in pixels) by adjusting the
number of lines and columns.
If FRAMES is nil, apply the font to the selected frame only.
If FRAMES is non-nil, it should be a list of frames to act upon,
or t meaning all graphical frames. Also, if FRAME is non-nil,
alter the user's Customization settings as though the
font-related attributes of the `default' face had been "set in
this session", so that the font is applied to future frames.