ErrorTracker.Web.Layouts uses `:live_view_socket`, but the example
shows `:socket` as the key.

This commit aligns the example with how it's used in the code. It looks like
`:socket` is not used anywhere in the codebase and does nothing. I was
able to get both longpolling and websockets working on custom paths
using `:live_view_socket`.

Since Elixir 1.18 there is a native `JSON` module, so new projects don't
need `Jason` and many don't have it. This pull request updates the error
tracker so:
- Jason is an optional dependency as the error tracker can work without
it
- We try to use the user-defined JSON library (just like we did before)
but if none is defined we default to the native `JSON` module for Elixir
1.18 and newer. On older versions we still default to `Jason` just like
we did before.
- We use the `encode_to_iodata!` function which is the most performant
and provided by both libraries.
- The `JSON` module can't pretty print like `Jason` does. Instead we are
now pretty printing in the client using `JSON.stringify` which is widely
available across all browsers.

There should not be any user-facing changes. Users will still see the
JSON context formatted just like they did before but now the
ErrorTracker works well on Elixir 1.18 and newer projects that don't
include Jason. We also don't force it for such projects as a native
alternative exists.