refactor(field): migrate ptdiff header to @sanity/ui

Author: vicmeow

packages/@sanity/field/src/types/portableText/diff/components/Header.css

@@ -1,5 +1,28 @@
+import {Heading} from '@sanity/ui'
 import React from 'react'
-import styles from './Header.css'
+import styled from 'styled-components'
+
+const headingSizes = {
+  h1: 2,
+  h2: 1,
+  h3: 0,
+  h4: 0,
+  h5: 0,
+  h6: 0,
+}
+
+const StyledHeading = styled(Heading)`
+  &:not([hidden]) {
+    display: inline;
+    text-transform: none;
+    margin: 0;
+
+    &::before,
+    &::after {
+      content: unset;
+    }
+  }
+`
 
 export default function Header({
   style,
@@ -8,5 +31,5 @@ export default function Header({
   style: string
   children: React.ReactNode
 }): JSX.Element {
-  return <div className={styles[style]}>{children}</div>
+  return <StyledHeading size={headingSizes[style]}>{children}</StyledHeading>
 }