remove findDOMNode from text-input (#1919)

## Summary:
`findDOMNode` is deprecated and React whines about it constantly when running tests, so I'm trying to chip away at removing uses of it.

Author: handeyeco

Reviewers: handeyeco, jeremywiebe

Required Reviewers:

Approved By: jeremywiebe

Checks: ✅ Publish npm snapshot (ubuntu-latest, 20.x), ✅ Lint, Typecheck, Format, and Test (ubuntu-latest, 20.x), ✅ Check for .changeset entries for all changed files (ubuntu-latest, 20.x), ✅ Cypress (ubuntu-latest, 20.x), ✅ Check builds for changes in size (ubuntu-latest, 20.x), ✅ Publish Storybook to Chromatic (ubuntu-latest, 20.x), ✅ gerald

Pull Request URL: #1919

.changeset/giant-tables-impress.md

@@ -0,0 +1,5 @@
+---
+"@khanacademy/perseus": patch
+---
+
+Remove usage of findDOMNode in text-input component

packages/perseus/src/components/text-input.tsx

@@ -1,7 +1,7 @@
 /* eslint-disable @khanacademy/ts-no-error-suppressions */
+import {Errors, PerseusError} from "@khanacademy/perseus-core";
 import {TextField} from "@khanacademy/wonder-blocks-form";
 import * as React from "react";
-import ReactDOM from "react-dom";
 
 import type {StyleType} from "@khanacademy/wonder-blocks-core";
 
@@ -31,6 +31,7 @@ function uniqueIdForInput(prefix = "input-") {
 }
 
 class TextInput extends React.Component<Props> {
+    inputRef = React.createRef<HTMLInputElement>();
     static defaultProps: DefaultProps = {
         value: "",
         disabled: false,
@@ -47,45 +48,46 @@ class TextInput extends React.Component<Props> {
         }
     }
 
+    _getInput: () => HTMLInputElement = () => {
+        if (!this.inputRef.current) {
+            throw new PerseusError(
+                "Input ref accessed before set",
+                Errors.Internal,
+            );
+        }
+
+        return this.inputRef.current;
+    };
+
     focus: () => void = () => {
-        // @ts-expect-error - TS2531 - Object is possibly 'null'. | TS2339 - Property 'focus' does not exist on type 'Element | Text'.
-        ReactDOM.findDOMNode(this).focus();
+        this._getInput().focus();
     };
 
     blur: () => void = () => {
-        // @ts-expect-error - TS2531 - Object is possibly 'null'. | TS2339 - Property 'blur' does not exist on type 'Element | Text'.
-        ReactDOM.findDOMNode(this).blur();
+        this._getInput().blur();
     };
 
     getValue: () => string | null | undefined = () => {
-        // @ts-expect-error - TS2339 - Property 'value' does not exist on type 'Element | Text'.
-        return ReactDOM.findDOMNode(this)?.value;
+        return this.inputRef.current?.value;
     };
 
     getStringValue: () => string | null | undefined = () => {
-        // @ts-expect-error - TS2339 - Property 'value' does not exist on type 'Element | Text'.
-        return ReactDOM.findDOMNode(this)?.value.toString();
+        return this.inputRef.current?.value.toString();
     };
 
     setSelectionRange: (arg1: number, arg2: number) => void = (
         selectionStart,
         selectionEnd,
     ) => {
-        // @ts-expect-error - TS2531 - Object is possibly 'null'. | TS2339 - Property 'setSelectionRange' does not exist on type 'Element | Text'.
-        ReactDOM.findDOMNode(this).setSelectionRange(
-            selectionStart,
-            selectionEnd,
-        );
+        this._getInput().setSelectionRange(selectionStart, selectionEnd);
     };
 
-    getSelectionStart: () => number = () => {
-        // @ts-expect-error - TS2531 - Object is possibly 'null'. | TS2339 - Property 'selectionStart' does not exist on type 'Element | Text'.
-        return ReactDOM.findDOMNode(this).selectionStart;
+    getSelectionStart: () => number | null = () => {
+        return this._getInput().selectionStart;
     };
 
-    getSelectionEnd: () => number = () => {
-        // @ts-expect-error - TS2531 - Object is possibly 'null'. | TS2339 - Property 'selectionEnd' does not exist on type 'Element | Text'.
-        return ReactDOM.findDOMNode(this).selectionEnd;
+    getSelectionEnd: () => number | null = () => {
+        return this._getInput().selectionEnd;
     };
 
     render(): React.ReactNode {
@@ -104,6 +106,7 @@ class TextInput extends React.Component<Props> {
 
         return (
             <TextField
+                ref={this.inputRef}
                 style={style}
                 disabled={disabled}
                 id={this.id}