auto-detect package manager in the plugin manager (#821)

* auto-detect package manager in the plugin installer

* linting

* linting

* memoize plugin manager

* fix linting issues

bridgetown-core/lib/bridgetown-core/plugin_manager.rb

@@ -129,6 +129,24 @@ def self.legacy_yarn_and_register(required_gems, skip_yarn: false)
                               "Required #{required_gems.map(&:name).join(", ")}")
     end
 
+    def self.package_manager
+      @package_manager ||= if File.exist?("yarn.lock")
+                             "yarn"
+                           elsif File.exist?("package-lock.json")
+                             "npm"
+                           elsif File.exist?("pnpm-lock.yaml")
+                             "pnpm"
+                           else
+                             ""
+                           end
+    end
+
+    def self.package_manager_install_command
+      package_manager == "npm" ? "install" : "add"
+    end
+
+    # rubocop:disable Metrics/PerceivedComplexity, Metrics/CyclomaticComplexity
+
     # Iterates through loaded gems and finds yard-add gemspec metadata.
     # If that exact package hasn't been installed, execute yarn add
     #
@@ -146,18 +164,22 @@ def self.install_yarn_dependencies(required_gems = bundler_specs, name: nil)
                          required_gems
                        end
 
+      # all right, time to install the package
       gems_to_search.each do |loaded_gem|
         yarn_dependency = find_yarn_dependency(loaded_gem)
         next unless add_yarn_dependency?(yarn_dependency, package_json)
 
-        # all right, time to install the package
-        cmd = "yarn add #{yarn_dependency.join("@")}"
+        next if package_manager.empty?
+
+        cmd = "#{package_manager} #{package_manager_install_command} #{yarn_dependency.join("@")}"
         system cmd
       end
 
       gems_to_search
     end
 
+    # rubocop:enable Metrics/PerceivedComplexity, Metrics/CyclomaticComplexity
+
     def self.find_yarn_dependency(loaded_gem)
       yarn_dependency = loaded_gem.to_spec&.metadata&.dig("yarn-add")&.match(YARN_DEPENDENCY_REGEXP)
       return nil if yarn_dependency&.length != 3 || yarn_dependency[2] == ""