New: Add fetch::missing::<resource-type> event

[alrra]

Ref: #960

[sarvaje]

When the resource is missing, I thing that most of the time we don't now the url/path

[alrra]

I forgot to change this to /** The URL of the target. */.

Also:

• resource this should be optional (for cases that do not have it)?
• Maybe this should also include an element also optional for cases such as <link rel="manifest" href="">? Or should that be another case?

[sarvaje]

resource this should be optional (for cases that do not have it)?

I think so.

[alrra]

Talking a bit with @molant and @sarvaje, and we decided to see in the future how to better address this if we event have to (most of the time there will probably be just one rule that will use this event).

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Note: I merged the improvements in c62ad58.