linux: Support ctrl-insert in markdown previews (#37273)

Closes: https://github.com/zed-industries/zed/issues/37240

Release Notes:

- Added support for copying in Markdown preview using `ctrl-insert` on Linux/Windows

assets/keymaps/default-linux.json

@@ -170,6 +170,7 @@
     "context": "Markdown",
     "bindings": {
       "copy": "markdown::Copy",
+      "ctrl-insert": "markdown::Copy",
       "ctrl-c": "markdown::Copy"
     }
   },
@@ -258,6 +259,7 @@
     "context": "AgentPanel > Markdown",
     "bindings": {
       "copy": "markdown::CopyAsMarkdown",
+      "ctrl-insert": "markdown::CopyAsMarkdown",
       "ctrl-c": "markdown::CopyAsMarkdown"
     }
   },