Detect the title of deck from first slide when title option is not passed

[yhatt]

It would be good to fill the title for converted HTML automatically from the heading of title slide when --title option (and title global directive) is not passed.

We make this plan from TODO comment in Marp CLI meta plugin.

https://github.com/marp-team/marp-cli/blob/5f6635af91ddd0f125c07c01b9a04e1a0c04864c/src/engine/meta-plugin.ts#L23

The other similar tool also has a plan for auto-filling of the title: https://github.com/jxnblk/mdx-deck/issues/103 (v3)