[Improvement]: Introduce langlib function count, first and last

[KavinduZoysa]

Description

Please check ballerina-platform/ballerina-spec#1135 (comment)

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-> Compilation

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[KavinduZoysa]

// lang.value
# Returns the number of arguments.
function count(any|error...) returns int;
@typeParam
type Type any|error;
# Returns the first argument.
function first(Type, any|error..) returns Type;
# Returns the last argument.
function last(Type, Type...) returns Type;