Improve Linux terminal keymap and context menu (#16845)

Follow-up https://github.com/zed-industries/zed/pull/16085 that fixes
the search deploy to be actually a part of the terminal-related
bindings.

Part of https://github.com/zed-industries/zed/issues/16839

Also 

* fixes few other bindings to use `shift` and avoid conflicts with the
existing key bindings.
* adds terminal inline assist to the context menu and makes both the
menu and the button to dynamically adjust to `assist.enabled` settings
change

It is still unclear to me, why certain labels for certain bindings are
wrong (it's still showing `ctrl-w` for closing the terminal tab, and
`shift-insert` instead of `ctrl-shift-v` for Paste, while Insert is near
and has a `ctrl-shift-c` binding shown) but at least the keys work now.

Release notes: 
- Improved Linux terminal keymap and context menu

assets/keymaps/default-linux.json

@@ -474,8 +474,7 @@
   {
     "context": "!Terminal",
     "bindings": {
-      "ctrl-shift-c": "collab_panel::ToggleFocus",
-      "ctrl-shift-f": "buffer_search::Deploy"
+      "ctrl-shift-c": "collab_panel::ToggleFocus"
     }
   },
   {
@@ -614,11 +613,15 @@
       "ctrl-alt-space": "terminal::ShowCharacterPalette",
       "ctrl-shift-c": "terminal::Copy",
       "ctrl-insert": "terminal::Copy",
-      // "ctrl-a": "editor::SelectAll", // conflicts with readline
       "ctrl-shift-v": "terminal::Paste",
       "shift-insert": "terminal::Paste",
       "ctrl-enter": "assistant::InlineAssist",
+      // Overrides for conflicting keybindings
       "ctrl-w": ["terminal::SendKeystroke", "ctrl-w"],
+      "ctrl-shift-a": "editor::SelectAll",
+      "ctrl-shift-f": "buffer_search::Deploy",
+      "ctrl-shift-l": "terminal::Clear",
+      "ctrl-shift-w": "pane::CloseActiveItem",
       "ctrl-e": ["terminal::SendKeystroke", "ctrl-e"],
       "up": ["terminal::SendKeystroke", "up"],
       "pageup": ["terminal::SendKeystroke", "pageup"],