Instead of picking up the first package manager found, require the user to specify their versioned file.

[dbanty]

This will enable specifying which file should be edited instead of the arbitrary "first" one being selected.

• When given a repo with all available metadata filetypes, only the selected one should be affected.
• No changelogs should be updated if not specified.
• When calling knope --generate, the first available metadata file (same order picked today) should be added as a package.
• When a PrepareRelease or BumpVersion step is called without a defined package—an error describing the recommended lines to add (the same thing --generate would output) should be returned to the user.
• More than one package specified should be an error pointing at Allow specifying multiple, independent packages. #153
• More than one file in the package should be an error pointing at Allow multiple versioned files in a normal repo (e.g., both a Cargo.toml and a pyproject.toml). #149
• Update the docs to indicate the [[packages]] pre-req for associated steps including details about how changelog works.
• Throw a useful error when the listed file in [[packages]] versioned_files is not a supported format