keymaps: Update defaults for inline assist and signature help (#39587)

Update the keybindings used in the default keymaps to better align with
VSCode's defaults, with the following changes:

* Windows & Linux
* `ctrl-enter` has been replaced by `ctrl-i` for
`assistant::InlineAssist`
* `ctrl-shift-space` maps to `editor::ShowSignatureHelp` instead of
`editor::ShowWordCompletions`
* MacOS
* `ctrl-enter` has been replaced by `cmd-i` for
`assistant::InlineAssist`
* `cmd-i` has been replaced by `cmd-shift-space` for
`editor::ShowSignatureHelp`

Closes #39278 

Release Notes:

- Changed the keybinding for `assistant: inline assist` from
`ctrl-enter` to `ctrl-i` for both Linux and Windows, and `cmd-i` for
MacOS. If you'd like to restore the old behavior, update your keymap
file with:
  ```
  {
    "context": "!ContextEditor > Editor && mode == full",
    "bindings": {
      "ctrl-enter": "assistant::InlineAssist"
    }
  }
  ```
- Changed the action dispatched by `ctrl-shift-space` from
`editor::ShowWordCompletions` to `editor::ShowSignatureHelp` on both
Linux and Windows. If you'd like to restore the old behavior, update
your keymap file with:
  ```  {
    "context": "Editor",
    "bindings": {
      "ctrl-shift-space": "editor::ShowWordCompletions"
    }
  }
  ```
- Changed the keybinding for `editor: show signature help` on MacOS from
`cmd-i` to `cmd-shift-space`. If you'd like to restore the old behavior,
update your keymap file with:
  ```  {
    "context": "Editor",
    "bindings": {
      "cmd-i": "editor::ShowSignatureHelp"
    }
  }
  ```

---------

Co-authored-by: Agus Zubiaga <agus@zed.dev>

assets/keymaps/default-linux.json

@@ -539,7 +539,7 @@
       "ctrl-k ctrl-0": "editor::FoldAll",
       "ctrl-k ctrl-j": "editor::UnfoldAll",
       "ctrl-space": "editor::ShowCompletions",
-      "ctrl-shift-space": "editor::ShowWordCompletions",
+      "ctrl-shift-space": "editor::ShowSignatureHelp",
       "ctrl-.": "editor::ToggleCodeActions",
       "ctrl-k r": "editor::RevealInFileManager",
       "ctrl-k p": "editor::CopyPath",
@@ -799,7 +799,7 @@
       "ctrl-shift-e": "pane::RevealInProjectPanel",
       "ctrl-f8": "editor::GoToHunk",
       "ctrl-shift-f8": "editor::GoToPreviousHunk",
-      "ctrl-enter": "assistant::InlineAssist",
+      "ctrl-i": "assistant::InlineAssist",
       "ctrl-:": "editor::ToggleInlayHints"
     }
   },
@@ -1093,7 +1093,7 @@
       "paste": "terminal::Paste",
       "shift-insert": "terminal::Paste",
       "ctrl-shift-v": "terminal::Paste",
-      "ctrl-enter": "assistant::InlineAssist",
+      "ctrl-i": "assistant::InlineAssist",
       "alt-b": ["terminal::SendText", "\u001bb"],
       "alt-f": ["terminal::SendText", "\u001bf"],
       "alt-.": ["terminal::SendText", "\u001b."],