Please associate debian/upstream/metadata with the yaml syntax

Hi,

Please have vim-debian associate the debian/upstream/metadata file with yaml format, such that out of the box it gets YAML syntax highlighting when available.

Most language servers also has a decent default for the YAML syntax and to improve that, I uploaded a schema for it to the schemastore. That is, for YAML tools that can use JSON schemas and fetch them from the schemastore, they can now see known fields and such.