Add extend_search_all command.

[DerSaidin]

This allows me to replicate my kakoune config:

  map global normal <a-%> '*%s<ret>'

With helix config:

  "A-%" = ["search_selection", "extend_search_all"]

[the-mikedavis]

This usecase is already possible with a similar macro keybinding:

[keys.normal]
"A-%" = "@*%s<ret>"