[hx] fix switch case (~)

[ikks]

Make ~ behave switching case:

• On selection switch case for all selected characters
• If none character was selected, replace the one below the cursors

[knightpp]

Fix https://codeberg.org/neurocyte/flow/pulls/17. But I wonder should there be two versions, one "switch selection or word under cursor" and helix variant "switch selection"?