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

assets/keymaps/vim.json

@@ -426,6 +426,7 @@
       ";": "vim::HelixCollapseSelection",
       ":": "command_palette::Toggle",
       "m": "vim::PushHelixMatch",
+      "s": "vim::HelixSelectRegex",
       "]": ["vim::PushHelixNext", { "around": true }],
       "[": ["vim::PushHelixPrevious", { "around": true }],
       "left": "vim::WrappingLeft",