Add shortcut to open note in a new split pane (PR #202) (#358)

This adds the shortcut `cmd/ctrl + alt + enter` to the search modal to open a found file in a new leaf. I used the term "split" in user-facing text, because I think that's the more common

Some things to note:
- This function ignores the existing "open in new pane by default" setting.
- It also does not check to see if the file is already open. It will continue opening the value in a new split as often as the user wants.

src/globals.ts

@@ -43,6 +43,7 @@ export const enum Action {
   ArrowDown = 'arrow-down',
   PrevSearchHistory = 'prev-search-history',
   NextSearchHistory = 'next-search-history',
+  OpenInNewLeaf = 'open-in-new-leaf',
 }
 
 export type DocumentRef = { path: string; mtime: number }