[items] Extend `metadata` & `itemchannellink` APIs

[proggprod]

See https://github.com/openhab/openhab-js/pull/212

I've been working with creating, updating , and deleting items with procedural scripts, and it would be much more powerful if sometimes I didn't have to specify item names or the precise metadata values etc. Sometimes I would like to delete all channel links from all items, or a selection of items, or a single item. Sometimes I would like to delete all invalid channel links.

[florian-h05]

What can be added is:

For metadata:

• remove all metadata from an Item
• get all metadata namespaces an Item has

For ItemChannelLinks:

• remove all links for an Item
• remove invalid/orphaned channel links

Doing such operations on all Items is possible by doing:

items.getItems().forEach((item) => item.someOperation);