feat($compile): Add .plain event modifier

Add a new event modifier `.plain` to check if event is "plain" (no shift/ctrl/meta/alt key is
pressed)

#5976

Author: nickmessing

src/compiler/codegen/events.js

@@ -31,7 +31,8 @@ const modifierCode: { [key: string]: string } = {
   meta: genGuard(`!$event.metaKey`),
   left: genGuard(`'button' in $event && $event.button !== 0`),
   middle: genGuard(`'button' in $event && $event.button !== 1`),
-  right: genGuard(`'button' in $event && $event.button !== 2`)
+  right: genGuard(`'button' in $event && $event.button !== 2`),
+  plain: genGuard(`$event.ctrlKey || $event.shiftKey || $event.altKey || $event.metaKey`)
 }
 
 export function genHandlers (

src/core/instance/proxy.js

@@ -27,7 +27,7 @@ if (process.env.NODE_ENV !== 'production') {
     Proxy.toString().match(/native code/)
 
   if (hasProxy) {
-    const isBuiltInModifier = makeMap('stop,prevent,self,ctrl,shift,alt,meta')
+    const isBuiltInModifier = makeMap('stop,prevent,self,ctrl,shift,alt,meta,plain')
     config.keyCodes = new Proxy(config.keyCodes, {
       set (target, key, value) {
         if (isBuiltInModifier(key)) {

test/unit/modules/compiler/codegen.spec.js

@@ -298,6 +298,10 @@ describe('codegen', () => {
       '<input @click.meta="onClick">',
       `with(this){return _c('input',{on:{"click":function($event){if(!$event.metaKey)return null;onClick($event)}}})}`
     )
+    assertCodegen(
+      '<input @click.plain="onClick">',
+      `with(this){return _c('input',{on:{"click":function($event){if($event.ctrlKey || $event.shiftKey || $event.altKey || $event.metaKey)return null;onClick($event)}}})}`
+    )
   })
 
   it('generate events with multiple modifiers', () => {