Move fish completion to vendor's directory

I noticed that task completions won't work recently but it can be
"fixed" by symlinking the completions scripts from vendor_completions.d
directory as described by fish docs [1].
Use this directory by default.

Fixes #3023

[1] https://fishshell.com/docs/current/completions.html

Author: Werkov

scripts/CMakeLists.txt

@@ -1,6 +1,12 @@
 cmake_minimum_required (VERSION 3.22...4.0)
-install (DIRECTORY bash fish vim hooks
+
+find_package(PkgConfig REQUIRED)
+pkg_get_variable(FISH_COMPLETIONSDIR fish completionsdir)
+
+install (DIRECTORY bash vim hooks
          DESTINATION ${TASK_DOCDIR}/scripts)
+install (FILES fish/task.fish
+         DESTINATION ${FISH_COMPLETIONSDIR})
 install (FILES zsh/_task
          DESTINATION share/zsh/site-functions)
 install (DIRECTORY add-ons