keymap_ui: Add open keymap JSON button (#36182)

Closes #ISSUE

Release Notes:

- Keymap Editor: Added a button in the top left to allow opening the
keymap JSON file. Right clicking the button provides shortcuts to
opening the default Zed and Vim keymaps as well.

crates/icons/src/icons.rs

@@ -140,6 +140,7 @@ pub enum IconName {
     Image,
     Indicator,
     Info,
+    Json,
     Keyboard,
     Library,
     LineHeight,