Exclude cursor when doing ctrl-w (helix-editor#2431)

Currently ctrl-w in insert mode deletes the cursor which results in
unexpected behavior. The patch also reduces the selection to cursor before
performing prev word to remove the behavior of removing unnecessary text
when nothing should be removed.

1. `::#(|)#::` after `ctrl-w` should be `#(|)#::`, previously `#(|)#:`
2. `#(|::)#` after `ctrl-w` should be `#(|::)#`, previously `#(|)#`

Fix helix-editor#2390

helix-term/src/commands.rs

@@ -2632,6 +2632,18 @@ pub mod insert {
     pub type Hook = fn(&Rope, &Selection, char) -> Option<Transaction>;
     pub type PostHook = fn(&mut Context, char);
 
+    /// Exclude the cursor in range.
+    fn exclude_cursor(text: RopeSlice, range: Range, cursor: Range) -> Range {
+        if range.to() == cursor.to() {
+            Range::new(
+                range.from(),
+                graphemes::prev_grapheme_boundary(text, cursor.to()),
+            )
+        } else {
+            range
+        }
+    }
+
     // It trigger completion when idle timer reaches deadline
     // Only trigger completion if the word under cursor is longer than n characters
     pub fn idle_completion(cx: &mut Context) {
@@ -2948,10 +2960,11 @@ pub mod insert {
         let (view, doc) = current!(cx.editor);
         let text = doc.text().slice(..);
 
-        let selection = doc
-            .selection(view.id)
-            .clone()
-            .transform(|range| movement::move_prev_word_start(text, range, count));
+        let selection = doc.selection(view.id).clone().transform(|range| {
+            let cursor = Range::point(range.cursor(text));
+            let next = movement::move_prev_word_start(text, cursor, count);
+            exclude_cursor(text, next, range)
+        });
         delete_selection_insert_mode(doc, view, &selection);
     }