fix: fix cursor position when cursor is at the end of the commandline (#9030)

# Description
Fix getting the cursor position, when it's at the end of the
commandline.

# User-Facing Changes
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helps us keep track of breaking changes. -->

# Tests + Formatting

# After Submitting

crates/nu-cli/src/commands/commandline.rs

@@ -118,8 +118,9 @@ impl Command for Commandline {
                     .expect("repl cursor pos mutex");
                 let char_pos = buffer
                     .grapheme_indices(true)
+                    .chain(std::iter::once((*cursor_pos, "")))
                     .position(|(i, _c)| i == *cursor_pos)
-                    .unwrap_or(buffer.len());
+                    .expect("Cursor position isn't on a grapheme boundary");
                 Ok(Value::String {
                     val: char_pos.to_string(),
                     span: call.head,