[Patch] Recognize "mouth" as optional member role in waterway relation

[hhtznr]

According to ​https://wiki.openstreetmap.org/wiki/Relation:waterway, the relation waterway can have an optional member mouth of object type node. The mouth is the place where the waterway ends (e.g. a node shared with another waterway or a sinkhole). So use of this member role is similar to using the role spring.

When adding a node member with the role mouth, JOSM warns that the tag is invalid. The attached small patch should make the role recognized.

[hhtznr]

Will this be considered?

[taylor.smock]

In 18997/josm:

Fix #23361: Add mouth as an optional role for waterway relations (patch by hhtznr)

Additional notes:

• The optional role was added to ​osmwiki:Relation:waterway on 2023-04-25.
• Of 104058 waterway relations, 845 have a mouth role.