Add Caption extension to JSON schema

[myl7]

Caption Python Markdown extension is introduced in the doc and implemented, but not defined in the JSON schema.
Users who follows the doc to add the schema in VSCode would find a warning on the value pymdownx.blocks.caption.
This PR adds the extension to the schema along with its options.

See also:

• https://squidfunk.github.io/mkdocs-material/setup/extensions/python-markdown-extensions/#caption
• https://facelessuser.github.io/pymdown-extensions/extensions/blocks/plugins/caption/

[squidfunk]

Thanks for the PR! This was a recent addition, so it was forgotten to update the schema.