Add whitelist mode

[mro95]

In this pull request, I've implemented a whitelist approach for publishing the notes.

This approach introduces an optional metadata keyword, publish, to the note. If publish: true is present in the metadata of the note, it will be included in the published site, otherwise it will be excluded. This option is useful for drafts, private notes, or content that isn't ready or appropriate for the public yet.

The new changes include:

• Adding a whitelist_mode field in the VaultBuilder structure to represent this behavior.
• Adding a new flag, whitelist_mode, to the Command enum (which will be enabled with the argument --whitelist-mode on the command line).
• Adding a new method, get, on the Metadata structure.
• Enhancing the note parsing loop in the VaultBuilder.build function to check each note's metadata for publish: true.

These changes don't affect the default behavior of obsidian-garden, only adding new functionality.

[ecarrara]

Thanks @mro95!

Do you mind adding a test case for this new flag --whitelist-mode?

[ecarrara]

You can use -t (or --tag) to publish notes with specific tag.

obsidian-garden -t public .   _build

But I agree, the tool should support the public: true/false property on front-matter section.