feat: open prompt in configured external editor (#7606)

Add `ctrl+g` shortcut to enable opening current prompt in configured
editor (`$VISUAL` or `$EDITOR`).


- Prompt is updated with editor's content upon editor close.
- Paste placeholders are automatically expanded when opening the
external editor, and are not "recompressed" on close
- They could be preserved in the editor, but it would be hard to prevent
the user from modifying the placeholder text directly, which would drop
the mapping to the `pending_paste` value
- Image placeholders stay as-is
- `ctrl+g` explanation added to shortcuts menu, snapshot tests updated



https://github.com/user-attachments/assets/4ee05c81-fa49-4e99-8b07-fc9eef0bbfce

codex-rs/Cargo.lock

@@ -1756,6 +1756,7 @@ dependencies = [
  "supports-color 3.0.2",
  "tempfile",
  "textwrap 0.16.2",
+ "thiserror 2.0.17",
  "tokio",
  "tokio-stream",
  "tokio-util",
@@ -1770,6 +1771,9 @@ dependencies = [
  "url",
  "uuid",
  "vt100",
+ "which",
+ "windows-sys 0.52.0",
+ "winsplit",
 ]
 
 [[package]]
@@ -8379,6 +8383,12 @@ version = "0.0.19"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "d135d17ab770252ad95e9a872d365cf3090e3be864a34ab46f48555993efc904"
 
+[[package]]
+name = "winsplit"
+version = "0.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3ab703352da6a72f35c39a533526393725640575bb211f61987a2748323ad956"
+
 [[package]]
 name = "wiremock"
 version = "0.6.5"