`lest.config.lua` is required for Lest to run

[Derpius]

Context

Scope

• Resort to a default config if the file is not found
  • In the future we’ll want to overhaul how options are loaded in general, but for now we should just focus on this bug. See Refactor how configuration options are loaded #102

Acceptance criteria

• When I run lest in a directory with no lest.config.lua, no error is thrown
• Additionally, it matches test files with the pattern *.test.lua if no config is found, and no CLI option is passed
  • Not sure if we have a CLI option yet. If we don’t, that can be ignored