Closes #56493

In Helix select mode, pressing `a` after a selection (e.g. `v a`) placed
the cursor one column too far to the right.

Bound `a` to `vim::HelixAppend` in the `helix_select` keymap so it
matches the behavior in `helix_normal`, and added a regression test for
the `v a` case.

Release Notes:

- Fixed cursor placement after pressing `a` in Helix select mode.

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Co-authored-by: Smit Barmase <heysmitbarmase@gmail.com>