Create syntax-nullish-coalescing-assignment.md

[consultart-artem]

Proposes adding ??= as a compound assignment operator that assigns only when the left-hand side is nil. Fills a gap in the existing compound assignment operator set (+=, -=, etc.). Addresses the common pattern of default value assignment where the or shorthand is unsafe for boolean values and the correct nil check form requires writing the variable path twice.

[aatxe]

in Luau, this would actually be written

config.Enabled = if config.Enabled == nil then true else config.Enabled

which does still repeat config.Enabled, of course, but it feels very weird to use a programming pattern we do not recommend for conditional code in an RFC.

[gaymeowing]

Is there a reason its ??= and not ?=? I would assume because of lookahead, but this isn't stated in the RFC.

[Bottersnike]

Will this run into the same problems safe navigation did? That is to say that Roblox userdata error on missing fields, rather than returning nil, making part.Foo ??= "bar" error in a surprising way?

To potentially answer my own question, I think it might be more okay than safe navigation ended up, because part.Foo = "bar" would error anyway, so the scope for problematic code is drastically reduced.