fix: Set PageMerge to default to get merged content for plain SVGs

[TeddyHuang-00]

Fixes #824

Explanation: The plain SVG target expected merged pages, while this is only true when the merge option is set in ExportSvgTask, so the simplest fix to this is to add a default PageMerge to the options.

This can be verified by adding the following test to simple-test.js:

console.assert(
  !!cc.plainSvg({
    mainFileContent: `
    #set page(width: auto, height: auto, margin: 0pt)

    Hello, Typst!
    `,
  }),
  'Simple test failed',
);

which, now with the fix, will pass.

P.S., it might be a good idea to expose this option to the node API, but that's a different topic and requires consideration. The current fix just get the failing compilation to work.

[TeddyHuang-00]

Hi @Myriad-Dreamin, I know you're busy with other projects. This is just a gentle reminder that the CI is still awaiting your approval. Since GitHub doesn't always notify for new commits, I thought I'd reach out directly.

[Myriad-Dreamin]

@TeddyHuang-00 There are many backloged PRs in tinymist and I'm processing them. This PR looks good to me, and I will merge it quickly, but I may not make a typst.ts release in recent future :(.

[TeddyHuang-00]

There are many backloged PRs in tinymist and I'm processing them.

Totally understandable. Thank you for your awesome work, BTW.

I may not make a typst.ts release in recent future :(.

No worries. Since most people can just use svg over plainSvg, it shouldn't cause much issue.