Optimize the parsing ahead for deconstruction-declarations to avoid allocations

[jcouv]

        /// <summary>
        /// Check ahead for a deconstruction declaration. This requires at least one good-looking variable and the presence of an equals sign.
        /// Doesn't move the cursor.
        /// PROTOTYPE(tuples) Can this be done without allocations?
        /// </summary>
        private bool IsPossibleDeconstructionDeclaration()

in LanguageParser.cs

[jcouv]

The parsing ahead will be reduced after the new parsing for declaration expressions #15548