Add PNG exporter

[shigma]

Is your feature request related to a problem? Please describe. A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]

Typst officially supports PNG exporting (https://typst.app/docs/reference/png/), but typst.ts does not provide such API.

There is a previous issue https://github.com/Myriad-Dreamin/typst.ts/issues/165, but it doesn't seem to be exposed as a public method.

Describe the solution you'd like A clear and concise description of what you want to happen.

compiler.png(options)

Describe alternatives you've considered A clear and concise description of any alternative solutions or features you've considered.

Additional context Add any other context or screenshots about the feature request here.