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

test/test_go.rb

@@ -2582,6 +2582,7 @@ class TestGoFZF < TestBase
   def test_change_preview_window_rotate
     tmux.send_keys "seq 100 | #{FZF} --preview-window left,border-none --preview 'echo hello' --bind '" \
       "a:change-preview-window(right|down|up|hidden|)'", :Enter
+    tmux.until { |lines| assert(lines.any? { _1.include?('100/100') }) }
     3.times do
       tmux.until { |lines| lines[0].start_with?('hello') }
       tmux.send_keys 'a'