The lookahead parser accepts a parser and matches it but does not update the context when it succeeds.

Example

iex> alias Ergo.Context
iex> import Ergo.{Combinators, Terminals}
iex> parser = lookahead(literal("Hello"))
iex> assert %Context{status: :ok, ast: nil, input: "Hello World", index: 0} = Ergo.parse(parser, "Hello World")

iex> alias Ergo.Context
iex> import Ergo.{Combinators, Terminals}
iex> parser = lookahead(literal("Helga"))
iex> assert %Context{status: {:error, [{:lookahead_fail, {1, 4}, _}, {:bad_literal, {1, 4}, _}, {:unexpected_char, {1, 4}, _}]}, index: 3, col: 4, input: "lo World"} = Ergo.parse(parser, "Hello World")