Warn when normalization removes node (#4769)

* Warn when normalization removes node

Slate requires the invariant that children are all blocks or all inlines.
It enforces this in default normalization by removing children.
When such a node is removed, it's almost certainly due to a programming
error: the developer needs to fix their application to ensure it
maintains this invariant. But currently Slate does not tell the
developer this.

I made such a programming error, and spent a long time debugging nodes
that mysteriously went missing. I would have fixed it within 30 seconds
if Slate had warned me when it detected this error.

(Note I have used console.warn despite the eslint rule. As far as I can
see, Slate has no other facility for runtime warnings.)

* Add changeset

.changeset/wild-penguins-shout.md

@@ -0,0 +1,5 @@
+---
+'slate': patch
+---
+
+Warn when normalization removes node