Debian: install completion files

Install the shell completion files introduced in commit
8a980ca.

Testing on Linux Mint 20 (based on Ubuntu 18.04):

* bash: OK
* fish: OK
* zsh: OK. Did not work when completions were installed in
  /usr/share/zsh/site-functions/_quick-lint-js, so I had to customize
  the path.

Author: strager

completions/CMakeLists.txt

@@ -4,10 +4,16 @@
 cmake_minimum_required(VERSION 3.10)
 include(GNUInstallDirs)
 
+# zsh on Arch Linux and on Homebrew on macOS search in this path:
 install(
   FILES _quick-lint-js
   DESTINATION "${CMAKE_INSTALL_DATAROOTDIR}/zsh/site-functions"
 )
+# zsh on Linux Mint searches in this path:
+install(
+  FILES _quick-lint-js
+  DESTINATION "${CMAKE_INSTALL_DATAROOTDIR}/zsh/vendor-completions"
+)
 
 install(
   FILES quick-lint-js.bash

dist/debian/debian/quick-lint-js.install

@@ -1,2 +1,5 @@
 usr/bin usr
+usr/share/bash-completion usr/share
+usr/share/fish usr/share
 usr/share/man usr/share
+usr/share/zsh usr/share