fix(richtext-lexical): recursively unwrap generic Slate nodes in Lexical migration converter (#13202)

## What?

The Slate to Lexical migration script assumes that the depth of Slate
nodes matches the depth of the Lexical schema, which isn't necessarily
true. This pull request fixes this assumption by first checking for
children and unwrapping the text nodes.

## Why?

During my migration, I ran into a lot of copy + pasted rich text with
list items with untyped nodes with `children`. The existing migration
script assumed that since list items can't have paragraphs, all untyped
nodes inside must be text nodes.

The result of the migration script was a lot of invalid text nodes with
`text: undefined` and all of the content in the `children` being
silently lost. Beyond the silent loss, the invalid text nodes caused the
Lexical editor to unmount with an error about accessing `0 of
undefined`, so those documents couldn't be edited.

This additionally makes the migration script more closely align with the
[recursive serialization logic recommendation from the Payload Slate
Rich Text
documentation](https://payloadcms.com/docs/rich-text/slate#generating-html).

## Visualization

### Slate

```txt
Slate rich text content
┣━┳━ Unordered list
┋ ┣━┳━ List item
┋ ┋ ┗━┳━ Generic (paragraph-like, untyped with children)
┋ ┋   ┣━━━ Text (untyped) `Hello `
┋ ┋   ┗━━━ Text (untyped) `World!
[...]
```

### Lexical Before PR

```txt
Lexical rich text content (invalid)
┣━┳━ Unordered list
┋ ┣━┳━ List item
┋ ┋ ┗━━━ Invalid text (assumed the generic node was text, stopped processing children, cannot restore lost text without a restoring backup with Slate and rerunning the script after this MR)
[...]
```

### Lexical After PR

```txt
Lexical rich text content
┣━┳━ Unordered list
┋ ┣━┳━ List item
┋ ┋ ┣━━━ Text `Hello `
┋ ┋ ┗━━━ Text `World!
[...]
```

---------

Co-authored-by: German Jablonski <43938777+GermanJablo@users.noreply.github.com>

test/lexical/collections/RichText/generateSlateRichText.ts

@@ -73,7 +73,16 @@ export function generateSlateRichText() {
         {
           children: [
             {
-              text: "It's built with SlateJS",
+              // This node is untyped, because I want to test this scenario:
+              // https://github.com/payloadcms/payload/pull/13202
+              children: [
+                {
+                  text: 'This editor is built ',
+                },
+                {
+                  text: 'with SlateJS',
+                },
+              ],
             },
           ],
           type: 'li',