Add this factory method to create a single AstComputedField:

public static AstComputedField ComputedField(string name, AstExpression value) 

and this factory method to create multiple AstComputedField(s):

public static IEnumerable<AstComputedField> ComputedFields(params (string name, AstExpression value)[] fields)

Put this factory method in AstExpression along with all the other factory methods even though the return value is an AstComputedField instead of an AstExpression.