Fix return_diagnostics warning from Kernel.ParallelCompiler on Elixir 1.19+

[markszabo]

Running mix espec on Elixir 1.19 prints the following warning on every run, before any specs execute:

warning: you must pass return_diagnostics: true when invoking Kernel.ParallelCompiler functions
  (elixir 1.19.5) lib/kernel/parallel_compiler.ex:324: Kernel.ParallelCompiler.spawn_workers/3
  (espec 1.10.0) lib/mix/tasks/espec.ex:225: Mix.Tasks.Espec.parse_spec_files/2
  (espec 1.10.0) lib/mix/tasks/espec.ex:160: Mix.Tasks.Espec.run_espec/3
  (espec 1.10.0) lib/mix/tasks/espec.ex:140: Mix.Tasks.Espec.run/1
  (mix 1.19.5) lib/mix/task.ex:499: anonymous fn/3 in Mix.Task.run_task/5
  (mix 1.19.5) lib/mix/cli.ex:129: Mix.CLI.run_task/2

Elixir 1.19 made return_diagnostics: true mandatory for callers of Kernel.ParallelCompiler functions. The compile/2 call in lib/mix/tasks/espec.ex doesn't have this, and this PR adds it.

Backward compatibility

Safe on all Elixir versions:

• 1.19+ - fixes the warning
• 1.15–1.18 - return_diagnostics: true was introduced in 1.15.0 and silently accepted; it was only recommended, not required, so no warning was emitted
• < 1.15 - Kernel.ParallelCompiler.compile/2 accepts an open keyword list and reads options via Keyword.get. An unrecognised key is simply never read; no error or warning is raised

Though I only tested it on 1.19.

With return_diagnostics: true the third element of the return tuple changes shape, but espec's call-site only pattern-matches on {:error, _, _} and discards the third element, so this has no effect on behavior.

[zhabinka]

+1