Improve Editor::DuplicateSelection (#21976)

Improves the new `Editor::DuplicateSelection` @CharlesChen0823 added in
https://github.com/zed-industries/zed/pull/21154.

- Merge `duplicate_selection` and `duplicate_line` into single function.
- Add keyboard shortcuts to JetBrains and SublimeText keymaps.
- If the selection is empty (e.g. just a cursor) make
`Editor::DuplicateSelection` fallback to being the same as
`Editor::DuplicateLineDown`.
- Tested with multiple cursors and for multiple selections.

| Editor      | Action              | macOS       | Linux        |
| ----------- | ------------------- | ----------- | ------------ |
| VSCode      | Duplicate Selection |             |              |
| JetBrains   | Duplicate Selection | cmd-d       | ctrl-d       |
| XCode       | Duplicate           | cmd-d       | N/A          |
| SublimeText | duplicate_line      | cmd-shift-d | ctrl-shift-d |

This matches behavior of the `duplicate` functionality in all other
major editors, with one exception: other editors change the selection so
that the newly duplicated object, current Zed behavior leaves the
original selection unchanged (TODO?)

assets/keymaps/macos/jetbrains.json

@@ -11,7 +11,7 @@
       "ctrl->": "zed::IncreaseBufferFontSize",
       "ctrl-<": "zed::DecreaseBufferFontSize",
       "ctrl-shift-j": "editor::JoinLines",
-      "cmd-d": "editor::DuplicateLineDown",
+      "cmd-d": "editor::DuplicateSelection",
       "cmd-backspace": "editor::DeleteLine",
       "cmd-pagedown": "editor::MovePageDown",
       "cmd-pageup": "editor::MovePageUp",

assets/keymaps/macos/sublime_text.json

@@ -18,7 +18,7 @@
       "ctrl-shift-m": "editor::SelectLargerSyntaxNode",
       "cmd-shift-l": "editor::SplitSelectionIntoLines",
       "cmd-shift-a": "editor::SelectLargerSyntaxNode",
-      "cmd-shift-d": "editor::DuplicateLineDown",
+      "cmd-shift-d": "editor::DuplicateSelection",
       "ctrl-cmd-g": "editor::SelectAllMatches", // find_all_under
       "shift-f12": "editor::FindAllReferences",
       "alt-cmd-down": "editor::GoToDefinition",