feat: add touying-reducer for cetz and fletcher animation

[OrangeX4]

This is the touying-reducer function that Touying is planning to add. It allows decoupling Touying from specific packages dependencies, while still enabling easy support for pause and meanwhile animations for Cetz and Fletcher. Any suggestions on Touying's interface are welcome :-) @johannes-wolf @Jollywatt @fenjalien

#import "../lib.typ": *
#import "@preview/cetz:0.2.0"
#import "@preview/fletcher:0.4.1" as fletcher: node, edge

#let cetz-canvas = touying-reducer.with(reduce: cetz.canvas, cover: cetz.draw.hide)
#let fletcher-diagram = touying-reducer.with(reduce: (arr, ..args) => fletcher.diagram(..args, ..arr))

#let s = themes.metropolis.register(s, aspect-ratio: "16-9", footer: self => self.info.institution)
#let (init, slide) = utils.methods(s)
#show: init

// cetz animation
#slide[
  Cetz in Touying:

  #cetz-canvas({
    import cetz.draw: *

    rect((0,0), (5,5))

    (pause,)

    rect((0,0), (1,1))
    rect((1,1), (2,2))
    rect((2,2), (3,3))

    (pause,)

    line((0,0), (2.5, 2.5), name: "line")
  })
]

// fletcher animation
#slide[
  Fletcher in Touying:

  #fletcher-diagram(
    node-stroke: .1em,
    node-fill: gradient.radial(blue.lighten(80%), blue, center: (30%, 20%), radius: 80%),
    spacing: 4em,
    edge((-1,0), "r", "-|>", `open(path)`, label-pos: 0, label-side: center),
    node((0,0), `reading`, radius: 2em),
    edge((0,0), (0,0), `read()`, "--|>", bend: 130deg),
    pause,
    edge(`read()`, "-|>"),
    node((1,0), `eof`, radius: 2em),
    pause,
    edge(`close()`, "-|>"),
    node((2,0), `closed`, radius: 2em, extrude: (-2.5, 0)),
    edge((0,0), (2,0), `close()`, "-|>", bend: -40deg),
  )
]