chore: add exports util with tests (#1045)

[![PR App][icn]][demo] | Fix RM-XYZ
:-------------------:|:----------:

## 🧰 Changes
- Adds a util for parsing out the export names from an MDX string 
- Adds tests for a few different cases

## 🧬 QA & Testing

- [Broken on production][prod].
- [Working in this PR app][demo].


[demo]: https://markdown-pr-PR_NUMBER.herokuapp.com
[prod]: https://SUBDOMAIN.readme.io
[icn]:
https://user-images.githubusercontent.com/886627/160426047-1bee9488-305a-4145-bb2b-09d8b757d38a.svg

__tests__/lib/exports/index.test.ts

@@ -0,0 +1,19 @@
+import singleExportMdx from './input/singleExport.mdx?raw';
+import multipleExportsMdx from './input/multipleExports.mdx?raw';
+import weirdExportsMdx from './input/weirdExports.mdx?raw';
+import { exports } from '../../../lib';
+
+describe('export tags', () => {
+  it('returns a single export name', () => {
+
+    expect(exports(singleExportMdx)).toStrictEqual(['Foo']);
+  });
+  it('returns multiple export names', () => {
+
+    expect(exports(multipleExportsMdx)).toStrictEqual(['Foo', 'Bar']);
+  });
+  it('returns different types of export names', () => { 
+
+    expect(exports(weirdExportsMdx)).toStrictEqual(['Foo', 'bar', 'doSomethingFunction', 'YELLING']);
+  });
+});

__tests__/lib/exports/input/multipleExports.mdx

@@ -0,0 +1,12 @@
+import React from 'react';
+
+export const Foo = () => {
+  return <div>Hello World</div>;
+}
+
+export const Bar = () => {
+  return <Foo />;
+}
+
+## Hey there 
+<Foo />

__tests__/lib/exports/input/singleExport.mdx

@@ -0,0 +1,8 @@
+import React from 'react';
+
+export const Foo = () => {
+  return <div>Hello World</div>;
+}
+
+## Hey there 
+<Foo />

__tests__/lib/exports/input/weirdExports.mdx

@@ -0,0 +1,19 @@
+import React from 'react';
+
+export function Foo() {
+  return <div>Hello World</div>;
+}
+
+export const bar = () => {
+  return <Foo />;
+}
+
+export function doSomethingFunction(input) {
+  return input.trim();
+}
+
+export const 
+YELLING = () => {}
+
+## Hey there 
+<Foo />

__tests__/lib/hast.test.ts

@@ -1,4 +1,4 @@
-import { hast, hastFromHtml } from '../../lib';
+import { hast } from '../../lib';
 import { h } from 'hastscript';
 
 describe('hast transformer', () => {
@@ -19,6 +19,7 @@ describe('hast transformer', () => {
       h('h2', { id: 'its-coming-from-within-the-component' }, "It's coming from within the component!"),
     );
 
+    // @ts-ignore
     expect(hast(md, { components })).toStrictEqualExceptPosition(expected);
   });
 });