Add 'alt-bg' color for striped lines (#4370)

Test cases:

1. 'jump' should show alternating background colors even when 'alt-bg' is
not defined as before.

  go run main.go --bind load:jump

Two differences:
  * The alternating lines will not be in bold (was a bug)
  * The marker column will not be rendered with alternating background color

2. Use alternating background color when 'alt-bg' is set

  go run main.go --color bg:238,alt-bg:237
  go run main.go --color bg:238,alt-bg:237 --highlight-line

3. 'selected-bg' should take precedence

  go run main.go --color bg:238,alt-bg:237,selected-bg:232 \
                 --highlight-line --multi --bind 'load:select+up+select+up'

4. Should work with text with ANSI colors

  declare -f | perl -0777 -pe 's/^}\n/}\0/gm' |
    bat --plain --language bash --color always |
    go run main.go --read0 --ansi --reverse --multi \
                   --color bg:237,alt-bg:238,current-bg:236 --highlight-line

---

Close #4354
Fix #4372

CHANGELOG.md

@@ -1,6 +1,23 @@
 CHANGELOG
 =========
 
+0.62.0
+------
+- Added `alt-bg` color to create striped lines to visually separate rows
+  ```sh
+  fzf --color bg:237,alt-bg:238,current-bg:236 --highlight-line
+
+  declare -f | perl -0777 -pe 's/^}\n/}\0/gm' |
+    bat --plain --language bash --color always |
+    fzf --read0 --ansi --reverse --multi \
+        --color bg:237,alt-bg:238,current-bg:236 --highlight-line
+  ```
+- [fish] Improvements in CTRL-R binding (@bitraid)
+    - You can trigger CTRL-R in the middle of a command to insert the selected item
+    - You can delete history items with SHIFT-DEL
+- Bug fixes and improvements
+    - Fixed unnecessary 100ms delay after `reload`
+
 0.61.3
 ------
 - Reverted #4351 as it caused `tmux run-shell 'fzf --tmux'` to fail (#4559 #4560)