support yarn PnP

This release makes esmock compatible with yarn PnP. To do this, esmock is published with no dependencies and is instead bundled with resolvewithplus. Changes for this release,

• Changes the package-exported file to esmock.js from esmockLoader.js, a clerical change to make the "main" esmock file more obvious,
• Embeds the resolvewithplus package inside esmock, so the published package will have no dependencies and will support yarn PnP, thanks @koshic
• Detects and uses import.meta.resolve, rather than resolvewithlus, when it is provided by the host environment. See flag --experimental-import-meta-resolve
• Detects the presence of esmock's loader by using the loader directly. Previously, esmock tested process args with a regexp like this /--(experimental-)?loader[=\s,"']*esmock/.test(process.argv). Directly using the loader is more precise and allows other scripts to wrap and use esmock in un-anticipated ways, credit @koshic