Yarn (https://yarnpkg.com) completion support (#5542)

* yarn completion

plugins/yarn/yarn.plugin.zsh

@@ -0,0 +1,81 @@
+alias yi="yarn install"
+
+_yarn ()
+{
+    local -a _1st_arguments _dopts _dev _production
+    local expl
+    typeset -A opt_args
+
+    _dopts=(
+        '(--force)--force[This refetches all packages, even ones that were previously installed.]'
+    )
+
+    _installopts=(
+        '(--flat)--flat[Only allow one version of a package. On the first run this will prompt you to choose a single version for each package that is depended on at multiple version ranges.]'
+        '(--har)--har[Outputs an HTTP archive from all the network requests performed during the installation.]'
+        '(--no-lockfile)--no-lockfile[Don’t read or generate a yarn.lock lockfile.]'
+        '(--pure-lockfile)--pure-lockfile[Don’t generate a yarn.lock lockfile.]'
+    )
+
+    _dev=('(--dev)--dev[Save installed packages into the project"s package.json devDependencies]')
+
+    _production=('(--production)--production[Do not install project devDependencies]')
+
+    _1st_arguments=(
+      'help:Display help information about yarn' \
+      'init:Initialize for the development of a package.' \
+      'add:Add a package to use in your current package.' \
+      'install:Install all the dependencies listed within package.json in the local node_modules folder.' \
+      'publish:Publish a package to a package manager.' \
+      'remove:Remove a package that will no longer be used in your current package.' \
+      'cache:Clear the local cache. It will be populated again the next time yarn or yarn install is run.' \
+      'clean:Frees up space by removing unnecessary files and folders from dependencies.' \
+      'check:Verifies that versions of the package dependencies in the current project’s package.json matches that of yarn’s lock file.' \
+      'ls:List all installed packages.' \
+      'global:Makes binaries available to use on your operating system.' \
+      'info:<package> [<field>] - fetch information about a package and return it in a tree format.' \
+      'outdated:Checks for outdated package dependencies.' \
+      'run:Runs a defined package script.' \
+      'self-update:Updates Yarn to the latest version.' \
+      'upgrade:Upgrades packages to their latest version based on the specified range.' \
+      'why:<query> - Show information about why a package is installed.'
+    )
+    _arguments \
+    '*:: :->subcmds' && return 0
+
+    if (( CURRENT == 1 )); then
+        _describe -t commands "yarn subcommand" _1st_arguments
+        return
+    fi
+
+    case "$words[1]" in
+        add)
+        _arguments \
+        $_dopts \
+        $_dev \
+        $_production
+        ;;
+        install)
+        _arguments \
+        $_installopts \
+        $_dopts \
+        $_dev \
+        $_no_color \
+        $_production
+        ;;
+        update)
+        _arguments \
+        $_dopts
+        ;;
+        remove)
+        _arguments \
+        $_dopts
+        ;;
+        *)
+        _arguments \
+        ;;
+    esac
+
+}
+
+compdef _yarn yarn