deps: update peerDeps to support React 18 without warnings (#89)

- seems to work without any component modifications needed on React 18
  - React 18 was mostly _deprecations_ and new features per the upgrade guide: https://reactjs.org/blog/2022/03/08/react-18-upgrade-guide.html
  - React 18 has concurrent mode opt-in, which is technically breaking, but doesn't seem to effect this library (fortunately)

- ensure tests pass with React 18
  - update devDeps to React 18, react-dom 18, @types/react 18
  - update devDeps to RTL 13 which supports React 18
    - see previous commit for the migration from Enzyme to RTL since Enzyme never had official support for React 17, let alone React 18

- update example to use `createRoot` instead of `ReactDOM.render`
  - this is one of the opt-ins to React 18 mode, and doesn't seem to change anything about the example (just no warnings anymore)

Author: agilgur5

example/app.js

@@ -1,5 +1,5 @@
 import React, { Component } from 'react'
-import ReactDOM from 'react-dom'
+import { createRoot } from 'react-dom/client'
 
 import SignaturePad from '../src/index.js'
 
@@ -42,4 +42,4 @@ class App extends Component {
   }
 }
 
-ReactDOM.render(<App />, document.getElementById('container'))
+createRoot(document.getElementById('container')).render(<App />)

package-lock.json

@@ -50,16 +50,16 @@
   },
   "peerDependencies": {
     "prop-types": "^15.5.8",
-    "react": "0.14 - 17",
-    "react-dom": "0.14 - 17"
+    "react": "0.14 - 18",
+    "react-dom": "0.14 - 18"
   },
   "dependencies": {
     "signature_pad": "^2.3.2",
     "trim-canvas": "^0.1.0"
   },
   "devDependencies": {
     "@agilgur5/changelog-maker": "^3.0.0",
-    "@testing-library/react": "^12.1.5",
+    "@testing-library/react": "^13.3.0",
     "babel-core": "^6.26.3",
     "babel-eslint": "^10.0.2",
     "babel-jest": "^23.6.0",
@@ -72,8 +72,8 @@
     "jest": "^24.8.0",
     "jest-environment-jsdom-fifteen": "^1.0.2",
     "jest-without-globals": "^0.0.2",
-    "react": "^17.0.2",
-    "react-dom": "^17.0.2",
+    "react": "^18.2.0",
+    "react-dom": "^18.2.0",
     "react-hot-loader": "^4.13.0",
     "standard": "^13.0.2",
     "style-loader": "^0.13.1",