Show code snippet on SyntaxError and TokenMissingError

[josevalim]

Environment

• Elixir & Erlang/OTP versions (elixir --version): N/A
• Operating system: N/A

Current behavior

Today, SyntaxError and TokenMissingError do not show a code snippet.

Expected behavior

Show a code snippet like Erlang does:

$ erlc foo.erl
foo.erl:4:9: syntax error before: '*'
%    4|   321 + * 456.
%     |         ^

PRs welcome! Inspiration and suggestions from other languages are very welcome too.

[josevalim]

Closing in favor of #11332.