Improve errors on undefined variables.

[NduatiK]

This will help with better Elixir errors:

When annotations is false, we get no line number:

Annotations: true

[bcardarella]

@NduatiK is this still a valid issue?

[NduatiK]

I believe so, but it is now related to #50. If we reference an undefined variable in EEx we get the following:

  ~SHEET"""
  "color-number-" <> num do
    rule-1
    rule-2<%= number %>
  end
  ...

    error: undefined variable "number"
    │
  2 │   use LiveViewNative.Stylesheet, :mock
    │           ^^^^^^
    │
    └─ test/support/mock_sheet.ex:2:11: MockSheet.class/2

The error does not point us to the specific line with an undefined variable.

Same thing with RULES:

  def class("color-blue", target: :watch) do
    ~RULES"""
    rule-2<%= number %>
    """
  end

    error: undefined variable "number"
    │
  1 │ defmodule MockSheet do
    │           ^^^^^^
    │
    └─ test/support/mock_sheet.ex:1:11: MockSheet.class/2