feat: Add support for --melos

[olexale]

Changes proposed

With this PR I propose to add --melos and --no-melos flags which control whether spec should look for melos workspace. For backward compatibility, the default value is true.

This flag is extremely useful when a developer wants to focus on a single package development (--watch --coverage) and still be productive.

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[rrousselGit]

Could you expand on the reasoning behind this? I don't see the value quite yet

[olexale]

The project I'm working with has melos that controls 300+ packages, 2M+ lines of Dart code, and many tests. Running tests with plain old dart test and flutter test in all packages takes more than an hour.
Developers typically make changes in a single package at a time, but even though the changes are scoped, when using spec with --watch --coverage arguments, other packages are being tested in parallel. This makes our machines dead freeze in a few minutes.
Proposed changes make it possible to use spec on a single package that is a part of a melos workspace without processing the rest of the workspace.

[rrousselGit]

In that case, I would rather add arguments to filter the list of packages spec runs against, instead of enabling/disabling melos.

For example spec could take a list of paths, and exclude anything else that's not in those paths

[olexale]

Thanks, I'll evaluate the proposed solution. Closing this PR.