Add Message Reference (Linguini extension of Fluent)

[Ygg01]

Motivated by space-wizards/RobustToolbox#2273
Seeing discussion in:

• Allow all inline expressions as named arguments projectfluent/fluent#230
• Dynamic message references projectfluent/fluent#80

Problem

Let's say we have following message:

-cat = {$number ->
  *[one] Cat
  [other] Cats
}
-dog = {$number -> 
  *[one] Dog
  [other] Dogs
}

# We want message to be X attacked Y
attack-log-cat1-cat1 = Cat attacked cat.
attack-log-cat1-cat2 = Cat attacked cats.
attack-log-dog1-cat1 = Dog attacked cat.
attack-log-dog1-cat2 = Dog attacked cats.
# Etc.

This leads to combinatorial explosion. Instead we want something like

attack-log = { $$attacker(number: $atk_num) } attacked {$$defender(number: $def_num)}.

And then we pass following values into bundle

{
  "attacker": "-dog",
  "atk_num": 1,
  "defender": "-dog",
  "def_num": 2
}

Solution

I've decided to move the needle by:
A) Introducing a new fluent type called LinguiniReference
B) Introducing experimental syntax for start $$attacker which must be a Linguini reference. Plus allowing complex nesting like
$$attacker(number: $$something.Num) and access to attributes like so $$msg_id.Value
C) Create fast serialization of messages

Note: This is experimental syntax and will be probably changed depending on how it works out.