Add $FZF_DEFAULT_OPTS_FILE (#3618)

For those who prefer to manage default options in a file.
If the file is not found, fzf will exit with an error.

We're not setting a default value for it because:

1. it's hard to find a default value that can be universally agreed upon
2. to avoid fzf having to check for the existence of the file even when it's not used

CHANGELOG.md

@@ -12,6 +12,12 @@ CHANGELOG
     - You would wonder why fzf implements directory traversal anyway when it's a filter program following the Unix philosophy.
       But fzf has had [the traversal code for years][walker] to tackle the performance problem on Windows. And I decided to use the same approach on different platforms as well for the benefits listed above.
     - Built-in traversal is now done using the excellent [charlievieth/fastwalk][fastwalk] library, which easily outperforms its competitors and supports safely following symlinks.
+- Added `$FZF_DEFAULT_OPTS_FILE` to allow managing default options in a file
+    - See [#3618](https://github.com/junegunn/fzf/pull/3618)
+    - Option precedence from lower to higher
+        1. Options read from `$FZF_DEFAULT_OPTS_FILE`
+        1. Options from `$FZF_DEFAULT_OPTS`
+        1. Options from command-line arguments
 
 [find]: https://github.com/junegunn/fzf/blob/0.46.1/src/constants.go#L60-L64
 [walker]: https://github.com/junegunn/fzf/pull/1847