Add import and export buttons to the world info modal

[tjohnman]

Added buttons for exporting and importing the world info entries using the SillyTavern format to the world info modal.

For any parameters that are present in SillyTavern and not on MikuPad, it uses default values in the exported file, and ignores them when importing.

Exporting is done via a direct file download, while importing is done via a file chooser. When importing, if there are already entries in the session, the user is asked whether they want to replace them or import the new ones alongside the existing ones.

Partially addresses what is discussed in #41

[tjohnman]

I changed this to a draft because I think having a textarea unnecessary. I will implement importing via a file chooser instead.

[tjohnman]

Since I could not see a mechanism to pass data from one modal to the other, I saved the JSON being imported in a global state object. Perhaps there could be a way to pass parameters to a modal via toggleModal to avoid this.

[lmg-anon]

Since I could not see a mechanism to pass data from one modal to the other, I saved the JSON being imported in a global state object. Perhaps there could be a way to pass parameters to a modal via toggleModal to avoid this.

That's understandable, I'm also still not sure how to tackle this kind of problem, but I'm leaning towards thinking that a way to share state between modals would be the best approach.

Anyway, LGTM, Thank you for your contribution!