new committer (src): Lexi Winter (ivy)

per the committer's guide for new committers:

• add my pgp key
• add myself to shared/authors.adoc
• add myself to shared/contrib-committers.adoc
• add a news entry

Reviewed by: kevans
Approved by: kevens (mentor)
Differential Revision: https://reviews.freebsd.org/D49901