CodingConventions: Prefer using const on immutable values.
Using const for values that should not be changed is considered a good practice (https://eslint.org/docs/rules/prefer-const).
Forum poll: https://wildfiregames.com/forum/topic/38347-codingconventions-use-const-instead-of-let-for-immutable-values-in-functions/
Differential revision: https://code.wildfiregames.com/D3950
Comments by: @bb, @Stan, @wraitii