The new keywords from metadata to be added to the header/footer template.

[macchaka]

Metadata(yaml header) is ignored. It is not displayed.
It is mean that vscode-markdown-pdf can aware that it is metadata.

one step futher,
I hope be able to use at header, footer and pdf properties.

e.g.

---
title: Some title
version: 5
---

%%title%% will replaced to Some title.
%%ISO-DATE%%(ver.%%version%%) will replaced to 2024-05-11(ver.5).

I hope that When enclosed in %%, it lookup to metadata definition.