Add overrideFile to avoid calling getActiveFile

[matthewturk]

I would like to call this plugin from a meta-bind button inside a sidecar-panel file. It turns out that for reasons related to how I need to render the meta-bind embeds, the getActiveFile() function won't produce anything useful. So I added this argument to try to make it optional, so that I can call it like this:

 ```meta-bind-button
 label: Export to PDF
 icon: ""
 hidden: false
 class: ""
 tooltip: ""
 id: ""
 style: primary
 actions:
   - type: inlineJS
     code: >
       const sidecar = app.plugins.plugins["contextual-sidecar"];
       const pandoc = app.plugins.plugins["obsidian-pandoc"];
       const currentFile = await sidecar.getCurrentFile();
       const fullPath = app.vault.adapter.getFullPath(currentFile.path);
       const v = await pandoc.startPandocExport(fullPath, "pdf", "pdf", "PDF", currentFile);

I think this is the minimally invasive solution to it -- what do you think?

[matthewturk]

I've added a return value that is the actual generated filename. This lets it also utilize app.vault.resolveFilePath to grab the right path, and with meta-bind can embed.