Add environment variables: FZF_{BORDER,PREVIEW}_LABEL (#3693)

The environment variable get the value of the preview label, even if it
has been updated with an action. It can be useful to track the label of
the preview and be able to switch between previews using only one
binding.

Co-authored-by: Junegunn Choi <junegunn.c@gmail.com>

test/test_go.rb

@@ -2877,6 +2877,27 @@ class TestGoFZF < TestBase
     end
   end
 
+  def test_labels_variables
+    tmux.send_keys ': | fzf --border --border-label foobar --preview "echo \$FZF_BORDER_LABEL // \$FZF_PREVIEW_LABEL" --preview-label barfoo --bind "space:change-border-label(barbaz)+change-preview-label(bazbar)+refresh-preview,enter:transform-border-label(echo 123)+transform-preview-label(echo 456)+refresh-preview"', :Enter
+    tmux.until do
+      assert_includes(_1[0], '─foobar─')
+      assert_includes(_1[1], '─barfoo─')
+      assert_includes(_1[2], ' foobar // barfoo ')
+    end
+    tmux.send_keys :Space
+    tmux.until do
+      assert_includes(_1[0], '─barbaz─')
+      assert_includes(_1[1], '─bazbar─')
+      assert_includes(_1[2], ' barbaz // bazbar ')
+    end
+    tmux.send_keys :Enter
+    tmux.until do
+      assert_includes(_1[0], '─123─')
+      assert_includes(_1[1], '─456─')
+      assert_includes(_1[2], ' 123 // 456 ')
+    end
+  end
+
   def test_info_separator_unicode
     tmux.send_keys 'seq 100 | fzf -q55', :Enter
     tmux.until { assert_includes(_1[-2], '  1/100 ─') }