feat(typings): add key and ref to slot elements (#3177)

this pr adds the `key` and `ref` fields as optional onto the `slot`
element's typings. it also performs some minor refactoring to make
better reuse of typings.

src/declarations/stencil-public-runtime.ts

@@ -744,7 +744,7 @@ export namespace JSXBase {
     view: JSXBase.SVGAttributes;
   }
 
-  export interface SlotAttributes {
+  export interface SlotAttributes extends JSXAttributes {
     name?: string;
     slot?: string;
     onSlotchange?: (event: Event) => void;
@@ -1579,11 +1579,7 @@ export namespace JSXBase {
     zoomAndPan?: string;
   }
 
-  export interface DOMAttributes<T = Element> {
-    // vdom specific
-    key?: string | number;
-
-    ref?: (elm?: T) => void;
+  export interface DOMAttributes<T> extends JSXAttributes<T> {
     slot?: string;
     part?: string;
     exportparts?: string;
@@ -1731,6 +1727,12 @@ export namespace JSXBase {
   }
 }
 
+export interface JSXAttributes<T = Element> {
+  // vdom specific
+  key?: string | number;
+  ref?: (elm?: T) => void;
+}
+
 export interface CustomElementsDefineOptions {
   exclude?: string[];
   resourcesUrl?: string;