feat: add ability to check if a plugin has been installed

plugins/plugin/internal-functions

@@ -12,6 +12,15 @@ cmd-plugin-list() {
   plugn list
 }
 
+cmd-plugin-installed() {
+  declare desc="checks if a plugin is installed"
+  declare cmd="plugin:installed"
+  [[ "$1" == "$cmd" ]] && shift 1
+  declare PLUGIN_NAME="$1"
+
+  plugn list | awk '{print $1}' | grep -q "^${PLUGIN_NAME}$"
+}
+
 plugin_prime_bash_completion() {
   declare desc="primes the bash-completion cache"
 

plugins/plugin/subcommands/installed

@@ -0,0 +1,6 @@
+#!/usr/bin/env bash
+set -eo pipefail
+[[ $DOKKU_TRACE ]] && set -x
+source "$PLUGIN_AVAILABLE_PATH/plugin/internal-functions"
+
+cmd-plugin-installed "$@"