Elixir 1.13 support

[lukaszsamson]

TODO:

• bump elixir_sense
• crash in debugger test (see Crash on elixir-LS testsuite and broken test output elixir-lang/elixir#11371) (extracted to Debugger tests are crashing on 1.13 #627)
• failing test test/providers/execute_command/expand_macro_test.exs:49
• failing test test/providers/folding_range_test.exs:464
• failing test test/server_test.exs:947
• failing test test/source_file_test.exs:700 (see Change in path parsing in URI.parse() elixir-lang/elixir#11363)
• failing test test/providers/code_lens/type_spec/contract_translator_test.exs:146
• breaking change in Mix.Shell behaviour (see Breaking change in Mix.Shell.yes? elixir-lang/elixir#11372)
• Mix.Config.persist/1 hard deprecated. Replace with Application.put_all_env(config, persistent: true)
• Macro.to_string/2 is deprecated (see Macro.to_string/2 hard deprecated with no suitable replacement elixir-lang/elixir#11373) (not a problem for now)
• Mix.Config hard deprecated (https://hexdocs.pm/elixir/master/Config.html#module-migrating-from-use-mix-config)
• newline characters in document outline
• docsh crashing on OTP24 (not a problem)

Edit:
Elixir 1.13-rc.1 is out with fixes for regressions found so far. I fixed all outstanding issues in elixir-ls. I think we can merge it now and get some more testing from the community.

[axelson]

Looks great, thanks for all your work on this! ❤️