feat: add speaker note support for dual screens

[OrangeX4]

#let s = (s.methods.show-notes-on-second-screen)(self: s, right)

#speaker-note[
  + This is a speaker note.
  + You won't see it unless you use `#let s = (s.math.show-notes-on-second-screen)(self: s, right)`
]

Add speaker note support for dual screens. It is compatible with beamer, so we can use dspdfviewer or pympress for presentation with speaker notes. I personally recommend pympress because of its feature-rich and simple installation.

touying/examples/example.typ

Lines 85 to 88 in dc0910b

	#speaker-note[
	+ This is a speaker note.
	+ You won't see it unless you use `#let s = (s.math.show-notes-on-second-screen)(self: s, right)`
	]

example.pdf

cc @cooki35 @zouharvi @JuliusFreudenberger, maybe @laurmaedje for presentation mode for typst.app

Hope to have more feedback, thanks :-)