Fix UTF-8 multibyte handling in explore inputs (#16325)

# Description
`explore` was reading the byte length for computing:
- `:` command input cursor positions
- `/`/`?` search mode cursor positions
- `:try` input box cursor positions
- search highlighting

Fixed this for the majority of cases by using `unicode_width`, this is
only best effort as terminals don't need to follow those expectations
but for the most cases (traditional language scripts etc.) this should
lead to better result. The only way around the uncertainty would be to
perform the highlighting/cursor marking as we go, but this may not be as
compatible with the `ratatui` framework.

Closes #16312

# User-Facing Changes
Fixed cursor position and search highlighting for non-ASCII characters,
with the caveat mentioned above.

# Tests + Formatting
Manually tested

crates/nu-explore/src/pager/mod.rs

@@ -37,6 +37,7 @@ use std::{
     io::{self, Stdout},
     result,
 };
+use unicode_width::UnicodeWidthStr;
 
 pub type Frame<'a> = ratatui::Frame<'a>;
 pub type Terminal = ratatui::Terminal<CrosstermBackend<Stdout>>;
@@ -450,14 +451,14 @@ fn run_command(
 fn set_cursor_cmd_bar(f: &mut Frame, area: Rect, pager: &Pager) {
     if pager.cmd_buf.is_cmd_input {
         // todo: deal with a situation where we exceed the bar width
-        let next_pos = (pager.cmd_buf.buf_cmd2.len() + 1) as u16;
+        let next_pos = (pager.cmd_buf.buf_cmd2.width() + 1) as u16;
         // 1 skips a ':' char
         if next_pos < area.width {
             f.set_cursor_position((next_pos, area.height - 1));
         }
     } else if pager.search_buf.is_search_input {
         // todo: deal with a situation where we exceed the bar width
-        let next_pos = (pager.search_buf.buf_cmd_input.len() + 1) as u16;
+        let next_pos = (pager.search_buf.buf_cmd_input.width() + 1) as u16;
         // 1 skips a ':' char
         if next_pos < area.width {
             f.set_cursor_position((next_pos, area.height - 1));
@@ -558,7 +559,8 @@ fn render_cmd_bar_search(f: &mut Frame, area: Rect, pager: &Pager<'_>, config: &
 
 fn render_cmd_bar_cmd(f: &mut Frame, area: Rect, pager: &Pager, config: &ExploreConfig) {
     let mut input = pager.cmd_buf.buf_cmd2.as_str();
-    if input.len() > area.width as usize + 1 {
+    // UnicodeWidthStr::width is a best guess
+    if input.width() > area.width as usize + 1 {
         // in such case we take last max_cmd_len chars
         let take_bytes = input
             .chars()
@@ -591,7 +593,8 @@ fn highlight_search_results(f: &mut Frame, pager: &Pager, layout: &Layout, style
         if let Some(p) = text.find(&pager.search_buf.buf_cmd_input) {
             let p = covert_bytes_to_chars(&text, p);
 
-            let w = pager.search_buf.buf_cmd_input.len() as u16;
+            // this width is a best guess
+            let w = pager.search_buf.buf_cmd_input.width() as u16;
             let area = Rect::new(e.area.x + p as u16, e.area.y, w, 1);
 
             f.render_widget(highlight_block.clone(), area);

crates/nu-explore/src/views/try.rs

@@ -16,6 +16,7 @@ use ratatui::{
     widgets::{BorderType, Borders, Paragraph},
 };
 use std::cmp::min;
+use unicode_width::UnicodeWidthStr;
 
 pub struct TryView {
     input: Value,
@@ -87,8 +88,8 @@ impl View for TryView {
 
         let mut input = self.command.as_str();
 
-        let max_cmd_len = min(input.len() as u16, cmd_input_area.width);
-        if input.len() as u16 > max_cmd_len {
+        let max_cmd_len = min(input.width() as u16, cmd_input_area.width);
+        if input.width() as u16 > max_cmd_len {
             // in such case we take last max_cmd_len chars
             let take_bytes = input
                 .chars()