Add <c-w> and <a-d> (along with <c-W> and <a-D> that work on WORDs),
and <c-y> which pastes the transient clipboard contant (which saves
big erase, such as word erase and line end/begin erase).
Fixes#2355
In the end, no better solution materialized so far, and custom
Kakoune line editing bindings are hard to remember. Using well
known readline bindings seems just more convenient.
Closes#800, although it does not contain all the binding proposed
by it (I might accept a few additional ones, such as <c-w>, but not
too much, I still see that as a hack pending a nicer solution).
Because keyboard layouts vary, the shift-modifier `<s-…>` is only supported
for special keys (like `<up>` and `<home>`) and for ASCII lowercase where
we assume the shift-modifier just produces the matching uppercase character.
Even that's not universally true, since in Turkish `i` and `I` are not an
uppercase/lowercase pair, but Kakoune's default keyboard mappings already
assume en-US mappings for mnemonic purposes.
Mappings of the form `<s-x>` are normalized to `<X>` when `x` is an ASCII
character. `<backtab>` is removed, since we can now say `<s-tab>`.
