rc windowing new-client: copy buffer and selections from calling client

"new" runs "kak -c" which opens a recently used buffer.  More often
than not, I rather want to preserve the calling client's buffer and
selections. For example I might select a word and run "new" followed by
"grep" on the selected word. If the selection is preserved I can use
"new grep".  A similar scenario is "new lsp-definition".

Make "new" use the calling client's buffer and selections to support
such scenarios. Center the viewport with vv, matching "edit <file>
<line>".

Vim does this in <c-w><c-v>.

Another scenario is when I want to look at two regions from a file,
side-by-side.

My initial patch used the code below but Zz seems better:

	echo "try %|
	    buffer -- '$(printf %s "$kak_buffile" | sed "s/'/''/g; s/|/||/g")'
	    select $kak_selections_desc
	    execute-keys vv
	|"

rc/windowing/new-client.kak

@@ -3,11 +3,22 @@ new [<commands>]: create a new Kakoune client
 The ''terminal'' alias is being used to determine the user''s preferred terminal emulator
 The optional arguments are passed as commands to the new client' \
 %{
-    terminal kak -c %val{session} -e %sh{
-        for arg
-        do
-            printf %s "'$(printf %s "$arg" | sed "s/'/''/g")' "
-        done
+    terminal kak -c %val{session} -e %exp{
+        %{
+            evaluate-commands -save-regs ^ %{
+                try %{
+                    execute-keys -client %val{client} -save-regs '' <esc>Z
+                    execute-keys z
+                    echo # clear message from z
+                }
+            }
+        }
+        %sh{
+            for arg
+            do
+                printf %s "'$(printf %s "$arg" | sed "s/'/''/g")' "
+            done
+        }
     }
 }