Export $FZF_KEY environment variable to child processes

It's the name of the last key pressed.

Related #3412

CHANGELOG.md

@@ -1,6 +1,28 @@
 CHANGELOG
 =========
 
+0.50.0
+------
+- Added `jump` and `jump-cancel` events that are triggered when leaving `jump` mode
+  ```sh
+  # Default behavior
+  fzf --bind space:jump
+
+  # Same as jump-accept action
+  fzf --bind space:jump,jump:accept
+
+  # Accept on jump, abort on cancel
+  fzf --bind space:jump,jump:accept,jump-cancel:abort
+
+  # Change header on jump-cancel
+  fzf --bind 'space:change-header(Type jump label)+jump,jump-cancel:change-header:Jump cancelled'
+  ```
+- Added a new environment variable `$FZF_KEY` exported to the child processes. It's the name of the last key pressed.
+  ```sh
+  fzf --bind 'space:jump,jump:accept,jump-cancel:transform:[[ $FZF_KEY =~ ctrl-c ]] && echo abort'
+  ```
+- Bug fixes
+
 0.49.0
 ------
 - Ingestion performance improved by around 40% (more or less depending on options)