feat(core): add default markdown behaviors to PTE

This change extends the core behaviors of PTE with default markdown behaviors
for a more pleasant writing experience. This allows you to:

1. Use `#` characters to create headings
2. Use `>` to create a blockquote
3. Use `Backspace` at the beginning of block to clear its style
4. Use `-`, `*`, `_` or `1.` to initiate a list

This is achieved by providing the `behaviors` configuration for PTE where the
`coreBehaviors` are first spread into the array (to preserve core behaviors)
and the additional markdown behaviors are added afterwards. The markdown
behaviors require us to define what styles to use, and here we just default to
the most common names for these. Later, we can figure out how to allow you to
configure markdown behaviors through the Schema, but for now these sensible
defaults are hard-coded.

The only downsides here is that:

1. You can't opt out of markdown behaviors if you for some reason don't like
   them
2. You can't change the configuration if for example your lists are named
   something else
3. Things could potentially get confusing if you use unconventional names for
   styles and lists, for example if `h1` means something completely different
   in your context.

These are downsides we'll have to live with for now.

packages/sanity/src/core/form/inputs/PortableText/PortableTextInput.tsx

@@ -14,6 +14,7 @@ import {
   useEditor,
   usePortableTextEditor,
 } from '@portabletext/editor'
+import {coreBehaviors, createMarkdownBehaviors} from '@portabletext/editor/behaviors'
 import {useTelemetry} from '@sanity/telemetry/react'
 import {isKeySegment, type Path, type PortableTextBlock} from '@sanity/types'
 import {Box, Flex, Text, useToast} from '@sanity/ui'
@@ -384,6 +385,21 @@ export function PortableTextInput(props: PortableTextInputProps): ReactNode {
           <PortableTextMemberItemsProvider memberItems={portableTextMemberItems}>
             <EditorProvider
               initialConfig={{
+                behaviors: [
+                  ...coreBehaviors,
+                  ...createMarkdownBehaviors({
+                    defaultStyle: ({schema}) =>
+                      schema.styles.find((style) => style.value === 'normal')?.value,
+                    blockquoteStyle: ({schema}) =>
+                      schema.styles.find((style) => style.value === 'blockquote')?.value,
+                    headingStyle: ({schema, level}) =>
+                      schema.styles.find((style) => style.value === `h${level}`)?.value,
+                    orderedListStyle: ({schema}) =>
+                      schema.lists.find((list) => list.value === 'number')?.value,
+                    unorderedListStyle: ({schema}) =>
+                      schema.lists.find((list) => list.value === 'bullet')?.value,
+                  }),
+                ],
                 initialValue: value,
                 readOnly: readOnly || !ready,
                 keyGenerator,