The message key id for a setting's name, or undefined when the face has no key for it.
An array key such as SLOT[4] sits in message_keys under its base name alone, and each slot is
the base id plus its index, so SLOT[1] is looked up as SLOT plus one.
Parameters
messageKeys: any
The face's message_keys map.
name: string
The setting's name, such as CLOCK_DATE_FORMAT or SLOT[1].
The message key id for a setting's name, or undefined when the face has no key for it.
An array key such as SLOT[4] sits in message_keys under its base name alone, and each slot is the base id plus its index, so SLOT[1] is looked up as SLOT plus one.