Add `package-json-path` input to specify location of `package.json`

[rwjblue]

For projects without a package.json in the working directory root we currently do not support the ability to pin specific node-version | yarn-version | npm-version (we are only able to ensure that the given versions are preinstalled, but when node|yarn|npm are ultimately ran they might not use the specified). This is because we are only able to pin if we can find the package.json which contains the volta property (which we need to override).

This adds a new input package-json-path that can be used to specify where in a project the package.json is actually located. When not specified, we will use package.json (e.g. a package.json in the root of the working directory).

Closes #85

[shamrin]

Thank you @rwjblue and @scalvert!