Improve the file finder picker footer design (#31777)

The current filter icon button in the file finder picker footer confused
me because it is a really a toggleable button that adds a specific
filter. From the icon used, I was expecting more configuration options
rather than just one. Also, I was really wanting a way to trigger it
with the keyboard, even if I need my mouse to initially learn about the
keybinding.

So, this PR transforms that icon button into an actual popover trigger,
in which (for now) there's only one filter option. However, being a menu
is cool because it allows to accomodate more items like, for example,
"Include Git Submodule Files" and others, in the future. Also, there's
now a keybinding that you can hit to open that popover, as well as an
indicator that pops in to communicate that a certain item inside it has
been toggled.

Lastly, also added a keybinding to the "Split" menu in the spirit of
making everything more keyboard accessible!

| Before | After |
|--------|--------|
| ![CleanShot 2025-05-30 at 4  29
57@2x](https://github.com/user-attachments/assets/88a30588-289d-4d76-bb50-0a4e7f72ef84)
| ![CleanShot 2025-05-30 at 4  24
31@2x](https://github.com/user-attachments/assets/30b8f3eb-4d5c-43e1-abad-59d32ed7c89f)
|

Release Notes:

- Improved the keyboard navigability of the file finder filtering
options.

assets/keymaps/default-linux.json

@@ -928,6 +928,13 @@
       "tab": "channel_modal::ToggleMode"
     }
   },
+  {
+    "context": "FileFinder",
+    "bindings": {
+      "ctrl-shift-a": "file_finder::ToggleSplitMenu",
+      "ctrl-shift-i": "file_finder::ToggleFilterMenu"
+    }
+  },
   {
     "context": "FileFinder || (FileFinder > Picker > Editor) || (FileFinder > Picker > menu)",
     "bindings": {

assets/keymaps/default-macos.json

@@ -987,6 +987,14 @@
       "tab": "channel_modal::ToggleMode"
     }
   },
+  {
+    "context": "FileFinder",
+    "use_key_equivalents": true,
+    "bindings": {
+      "cmd-shift-a": "file_finder::ToggleSplitMenu",
+      "cmd-shift-i": "file_finder::ToggleFilterMenu"
+    }
+  },
   {
     "context": "FileFinder || (FileFinder > Picker > Editor) || (FileFinder > Picker > menu)",
     "use_key_equivalents": true,