feat: add scale arg for image export (#1263)

[KermanX]

This PR adds a new command line argument --scale:

slidev export --format png --scale 100
slidev export --format png --scale 0.1 --per-slide
slidev export --format md --scale 100

The internal of this feature is:

const context = await browser.newContext({
  // ...
  deviceScaleFactor: scale,
})

This can be a way to control the quality of the png export. (#1263)

Result

0.1x
2x