Add a `Distribution.editable` property

Recently, when trying to check if a distribution is editable, I noticed doing so using the current API is very un-ergonomic. This information is available via the direct_url.json file of PEP 610, which is exposed in Distribution.origin.

When url refers to a local directory, the dir_info key MUST be present as a dictionary with the following key:

editable (type: boolean): true if the distribution was installed in editable mode, false otherwise. If absent, default to false.

So, to check if the distribution is editable, we can check Distribution.origin.dir_info.editable, however, Distribution.origin might be None (no direct_url.json), and Distribution.origin.dir_info might not be set (url in direct_url.json in not a local directory), which.

is_editable = distribution.origin and distribution.origin.dir_info and distribution.origin.dir_info.editable

try:
    is_editable = distribution.origin.dir_info.editable
except AttributeError:
    is_editable = False

It would be helpful to have this information available via a more ergonomic API.

Start by tracing the Distribution API and its origin data, then review PEP 610's direct_url.json rules for dir_info and editable. Done means the new property exposes a boolean and returns false when origin or editable information is absent.