Improve Find/Replace shortcuts (#10297)

This PR changes ways the Find/Replace functionality in the
Buffer/Project Search is accessible via shortcuts. It makes those panels
work the same way as in VS Code and Sublime Text.

The details are described in the issue: [Make Find/Replace easier to
use](https://github.com/zed-industries/zed/issues/9142)

There's a difficulty with the Linux keybindings:

VS Code uses on MacOS (this PR replicates it):

| Action | Buffer Search | Project Search |
| --- | --- | --- |
| Find | `cmd-f` | `cmd-shift-f` |
| Replace | `cmd-alt-f` | `cmd-shift-h` |

VS Code uses on Linux (this PR replicates all but one):

| Action | Buffer Search | Project Search |
| --- | --- | --- |
| Find | `ctrl-f` | `ctrl-shift-f` |
| Replace | `ctrl-h` ❗ | `ctrl-shift-h` |

The problem is that `ctrl-h` is already taken by the `editor::Backspace`
action in Zed on Linux.

There's two options here:

1. Change keybinding for `editor::Backspace` on Linux to something else,
and use `ctrl-h` for the "replace in buffer" action.
2. Use some other keybinding on Linux in Zed. This PR introduces
`ctrl-r` for this purpose, though I'm not sure it's the best choice.

What do you think?

fixes #9142

Release Notes:

- Improved access to "Find/Replace in Buffer" and "Find/Replace in
Files" via shortcuts (#9142).

Optionally, include screenshots / media showcasing your addition that
can be included in the release notes.

- N/A

crates/search/src/search.rs

@@ -22,6 +22,7 @@ actions!(
     search,
     [
         CycleMode,
+        FocusSearch,
         ToggleWholeWord,
         ToggleCaseSensitive,
         ToggleIncludeIgnored,