Warn about unknown server property keys at run

[afrigon]

• mc run prints a warning for each key in the manifest's server > properties block that is not a known server property; the check happens in to_entries, against the base map it already builds from the ServerProperties defaults
• the key is still written, since mods read their own keys from the file
• unit test on the to_entries result, docs in the manifest reference and the mc run page

Closes #23