feat: implement modular modal editor and isolated Ruby plugins

examples/plugins/greeting.rb

@@ -0,0 +1,12 @@
+Vamp.plugin "greeting", requires: ["text"]
+
+Vamp.command "greet" do |editor, args|
+  name = args.empty? ? "world" : args
+  editor.insert "Hello, #{name}!\n"
+  editor.message "Greeting inserted by Ruby"
+end
+
+Vamp.command "upper" do |editor, _args|
+  editor.replace editor.text.upcase
+  editor.message "Buffer uppercased by Ruby"
+end