Support relative paths for Bin hooks

[charlespierce]

Closes #459

Info

• The Hooks configuration has an option bin that allows you to specify a command to run to resolve the hook.
• Currently, that command is run relative to the current directory, so if you specify a relative path in hooks.json, it will only work if you are in the correct directory.
• The easiest way to specify hooks will be with a relative path to the script, so we should support this use-case.

Changes

• Updated the hooks deserialization to propagate the base directory that the hooks.json file is found in to any Bin hooks.
• If the command is detected as being a relative path (if it begins with .), then we execute it relative to the base directory found above.
• Added detection for non-zero exit codes, so that if the hook command returns a failure code, we throw a Volta error instead of continuing as if the command succeeded.
• Also updated stderr to inherit from the parent process, so that bin hooks can use it to show progress bars or waiting spinners, as well as report errors directly to the user.

Tested

• Verified that a bin hook with a relative path correctly executes anywhere in a project, as opposed to only from a single directory.

[chriskrycho]

Looks good overall! Had one small thing which needs a tweak; otherwise 👍

[chriskrycho]

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