The digit/0 parser accepts a character in the range of 0..9

Examples

iex> alias Ergo.Context
iex> import Ergo.Terminals
iex> parser = digit()
iex> assert %Context{status: :ok, ast: ?0, input: "000", index: 1, line: 1, col: 2} = Ergo.parse(parser, "0000")

iex> alias Ergo.Context
iex> import Ergo.Terminals
iex> import Ergo.Terminals
iex> parser = digit()
iex> assert %Context{status: {:error, [{:unexpected_char, {1, 1}, "Expected: |0|..|9| Actual: |A|"}]}, input: "AAAA", index: 0, line: 1, col: 1} = Ergo.parse(parser, "AAAA")

iex> alias Ergo.{Context, Parser}
iex> import Ergo.Terminals
iex> ctx = Context.new("")
iex> parser = digit()
iex> assert %Context{status: {:error, [{:unexpected_eoi, {1, 1}, "Unexpected end of input"}]}, input: "", index: 0, line: 1, col: 1} = Parser.invoke(ctx, parser)