Structs should raise Protocol.UndefinedError when used as context in Solid.render!/3

Hi! I'm making this issue to discuss updating the response for unsupported types in the Solid.Matcher protocol. I encountered this issue when passing a struct into `Solid.render!/3`, using the `[strict_filters: true, strict_variables: true]` filters. The function was returning an `{:error, :not_found}` response, which was a little confusing since maps **are** a natively implemented type in Solid, and [Elixir treats structs as maps under the hood](https://hexdocs.pm/elixir/structs.html#structs-are-bare-maps-underneath). I was able to encounter a similar error by replacing the string keys in a map with `:atom` keys, leading me to believe the issue was due to the expectation that maps would adhere to the `%{"key" => "value"}` syntax, rather than the `%{key: :value}` syntax found in structs due to how variables are passed as strings into `Map.fetch/2`. I totally agree (after "RTFM"-ing) that the implementing application should handle how a proprietary struct implements the `Solid.Matcher` protocol, but I think it would've been more helpful to receive a `**Protocol.UndefinedError` message during development, instead of `1: Undefined variable`. This matches native Elixir patterns seen in situations like:
```elixir
1 |> Enum.map(& &1)
# ** (Protocol.UndefinedError) protocol Enumerable not implemented for 1 of type Integer
# (elixir 1.17.2) lib/enum.ex:1: Enumerable.impl_for!/1
# (elixir 1.17.2) lib/enum.ex:166: Enumerable.reduce/3
# (elixir 1.17.2) lib/enum.ex:4423: Enum.map/2
```

I have a local branch with this behavior tested and ready for a PR if this behavior is worth implementing.

Thanks!

Start by reading the Solid.Matcher protocol and the Solid.render!/3 path when strict_filters and strict_variables are enabled. Reproduce the struct and atom-key map cases, then compare the current {:error, :not_found} or undefined-variable response with the requested Protocol.UndefinedError behavior and verify the existing local tests cover it.

Written by the indexing model from the issue text.