WIP: base: Configure substitution's "ignore missing" on notification messages

TextDocument substitution API already supported an ignore_missing argument, so that we can programmatically control whether missing entries should be rendered as ${variable} or should raise an error. This makes sense for some "important" notifications, where not sending a message would be less problematic than sending a message where some variables have not been substituted.

This extends this concept by allowing to configure as a property on the notification message whether missing entries should be ignored. This default to "ignore" to maintain compatibility.