Missing position info when using `keepPosition`

[Snack-X]

When I provide keepPosition: true to createMarkdownParser, position data on first node will be missing.

https://github.com/nuxt-modules/mdc/blob/a69242835d10733cfa8a9149dd74c47ea75b92df/src/runtime/parser/compiler.ts#L20-L25

node.position?.start?.offset will always be falsy for offset 0.