snap: add assets verbatim (#80585)

* snap: add assets verbatim

Providing the desktop files in snap/gui will have snapcraft copy over
those assets into the snaps meta/gui verbatim.

Addtionally, add a leading ${SNAP} to the Icon entry which was also
moved to snap/gui for the "package" to have an icon by default
(which is important when sideloading).

Signed-off-by: Sergio Schvezov <sergio.schvezov@canonical.com>

* escape

* fix snapcraft.yml

* use @@EXEC@@ in desktop files

* fix bad path

* fix snap desktop file

resources/linux/code-url-handler.desktop

@@ -2,7 +2,7 @@
 Name=@@NAME_LONG@@ - URL Handler
 Comment=Code Editing. Redefined.
 GenericName=Text Editor
-Exec=/usr/share/@@NAME@@/@@NAME@@ --open-url %U
+Exec=@@EXEC@@ --open-url %U
 Icon=@@ICON@@
 Type=Application
 NoDisplay=true