Introduce a New assistant: insert into editor Action (#13467)

This implements the functionality (paired with @as-cii), but we weren't
sure what the clearest name would be for the action. It's essentially
the inverse of "quote selection" - but what's the opposite of quoting
the selection?

One idea:
* Rename "quote selection" to "Insert **into** assistant"
* Name this "Insert **from** assistant"

Release Notes:

- Added action to insert from assistant into editor (default keybinding:
`cmd-<` on macOS, `ctrl-<` on Linux)

---------

Co-authored-by: Antonio Scandurra <me@as-cii.com>
Co-authored-by: Bennet <bennet@zed.dev>

assets/keymaps/default-linux.json

@@ -155,6 +155,7 @@
       //   }
       // ],
       "ctrl->": "assistant::QuoteSelection",
+      "ctrl-<": "assistant::InsertIntoEditor",
       "ctrl-alt-e": "editor::SelectEnclosingSymbol"
     }
   },
@@ -548,6 +549,7 @@
       "ctrl-enter": "assistant::Assist",
       "ctrl-s": "workspace::Save",
       "ctrl->": "assistant::QuoteSelection",
+      "ctrl-<": "assistant::InsertIntoEditor",
       "shift-enter": "assistant::Split",
       "ctrl-r": "assistant::CycleMessageRole",
       "enter": "assistant::ConfirmCommand",