1691759 - Add variable comment check for Fluent linter

Status: NEW

(Core :: Internationalization: Localization, enhancement)

Description

[Dan Minor [:dminor]]

For each variable in a message, there should be a comment explaining the purpose of the variable in a format like:

# $duration (Number) - The duration in seconds.
time-elapsed = Time elapsed: { NUMBER($duration, maximumFractionDigits: 0) }s.

It is possible to use group level comments (##) to explain variables for a group of messages.