add flow definition file for the element API

Author: koddsson

index.js.flow

@@ -0,0 +1,7 @@
+/* @flow strict */
+
+declare module '@github/markdown-toolbar-element' {
+  declare export default class MarkdownToolbarElement extends HTMLElement {
+    get field(): ?HTMLTextAreaElement;
+  }
+}

package.json

@@ -11,7 +11,7 @@
     "prebuild": "npm run clean && npm run lint && mkdir dist",
     "build-umd": "BABEL_ENV=umd babel index.js -o dist/index.umd.js",
     "build-esm": "BABEL_ENV=esm babel index.js -o dist/index.esm.js",
-    "build": "npm run build-umd && npm run build-esm",
+    "build": "npm run build-umd && npm run build-esm && cp index.js.flow dist/index.esm.js.flow && cp index.js.flow dist/index.umd.js.flow",
     "pretest": "npm run build",
     "test": "karma start test/karma.config.js",
     "prepublishOnly": "npm run build"