Add a 4th font button and an API to set its font

[tswwe]

Don't know why I can't create an issue in this repo. So I create a new issue at obsidian-excalidraw-plugin for potential further discussions. If I got something wrong or this is unneccessary, feel free to delete the issue.

[zsviczian]

I didn't have issues enabled for this repository. Now I do.
Btw, we can have a discussion in the PR directly.

[zsviczian]

so I notice that doing my edits on GitHub, I wasn't thorough enough.
App.tsx has two errors that I corrected after merging:
declaration for showFourthFont is correctly

export let showFourthFont: boolean = false;

And declaration of setLocalFont is correctly:

  public setLocalFont: ExcalidrawImperativeAPI["setLocalFont"] = (
    url: string,
    showOnPanel: boolean,
  ) => {
    this.setLocalFontUrl(url, showOnPanel);
  };

[tswwe]

I didn't have issues enabled for this repository. Now I do. Btw, we can have a discussion in the PR directly.

Yes, not did I recall that we can discuss directly in the PR before I submit it. Sorry for creating an unneccessary issue.

And thank you very much for improving the commits, making the functions much more robust and controllable.