Also trim trailing newlines when adding to history

When pasting and executing a full line, the trailing newline character
will be included in history.

I usually manually delete the newline before executing, but sometimes
I forget. When I recall my (typically single-line) commands, it's
surprising that the cursor is on the blank second line.

The newline doesn't seem useful. Let's remove it automagically.
I wonder if anyone will be thrown off by this smart behavior.

In future, we can make this space trimming configurable, similar to
fish_should_add_to_history.

src/reader.rs

@@ -5722,7 +5722,10 @@ fn add_to_history(&mut self) {
 
         // Historical behavior is to trim trailing spaces, unless escape (#7661).
         let mut text = self.command_line.text().to_owned();
-        while text.chars().next_back() == Some(' ')
+        while text
+            .chars()
+            .next_back()
+            .is_some_and(|c| matches!(c, ' ' | '\n'))
             && count_preceding_backslashes(&text, text.len() - 1) % 2 == 0
         {
             text.pop();