Implement regex_select action for Helix (#38736)

Closes #31561

Release Notes:

- Implemented the select_regex Helix keymap

Prior: The keymap `s` defaulted to `vim::Substitute`

After:
<img width="1387" height="376" alt="image"
src="https://github.com/user-attachments/assets/4d3181d9-9d3f-40d2-890f-022655c77577"
/>

Thank you to @ConradIrwin for pairing to work on this

crates/vim/src/state.rs

@@ -988,6 +988,7 @@ pub struct SearchState {
     pub prior_selections: Vec<Range<Anchor>>,
     pub prior_operator: Option<Operator>,
     pub prior_mode: Mode,
+    pub helix_select: bool,
 }
 
 impl Operator {