Add hook and API to transform tokens individually before rendering

[NathanLovato]

When parsing MDX documents, add a way to walk or retrieve tokens and call procedures to process them:

• Markdown image tags need to be converted into a PublicImage component with class names so there needs to be a way to create a new components, and replace a component with the new object
• Inline code sometimes need to be replaced with <IconGodot ...> components
• Video files need file paths to be make absolute
• Include components need to be replaced with the included and preprocessed content

For these tasks, the API should allow:

1. Looping over tokens
2. Editing tokens
3. Creating new tokens
4. Inserting tokens
5. Replacing tokens

For includes and special case, we can think of a simple token that contains text that gets inserted as-is? Preserving the original token indentation optionally