This commit adds support for rules with multiple tokens on LHS, also
known as "contraction rules", into stemmer override token
filter. Contraction rules are handy into translating multiple
inflected words into the same root form. One side effect of this change is
that it brings stemmer override rules format closer to synonym rules
format so that it makes it easier to translate one into another.
This change also makes stemmer override rules parser more strict so
that it should catch more errors which were previously accepted.
Closes#56113
a7c36c8af5