Warn on never-taken branch

[Dante-Broggi]

Is your feature request related to a problem? Please describe.

When translating a grammar from a grammar language with unordered choice (eg: BNF) it may happen that a simple transliteration results in branches being unable of being taken, simply due to being in an incorrect order for pest.
Eg:
X = { Identifier | Keyword }

This would also help if a rule is changed to be an option on an earlier rule:
Eg:

Item = { … | Statement | Declaration | … }
Statement = { … | Declaration /* new */ | … }

Describe the solution you'd like

It would be nice if the pest extension would warn if a latter branch is a strict subset of an earlier option in a rule.