Add preceding slash to subdirectory path

[misslivirose]

Previously, the extension would create a DownloadsMemoryCache folder instead of Downloads/MemoryCache. This fixed it, but I'm not sure if there's a better way.

[johnshaughnessy]

Hmm, it seems that the three ways to save have different requirements:

• saveAsPDF (silent mode): Runs your custom FF code. It requires a preceding slash to save to correct directory (as per this PR).
• saveAsPDF (non-silent mode): Automatically converts slashes to underscores, so we omit the subdirectory prefix all together.
• saveHtml (with browser.downloads.download): Disallows the preceding slash because it's interpreted as an absolute path.

So I recommend changing only the silent-mode saveAsPDF path (instead of DOWNLOAD_SUBDIRECTORY):

 async function savePDF() {
   try {
     await browser.tabs.saveAsPDF({
-      toFileName: `${DOWNLOAD_SUBDIRECTORY}/PAGE${generateFileName("pdf")}`,
+      toFileName: `/${DOWNLOAD_SUBDIRECTORY}/PAGE${generateFileName("pdf")}`,
       silentMode: true, // silentMode requires a custom build of Firefox
     });
   } catch (_e) {

Otherwise, the saveHtml path breaks: