This commit adds a `isValidUrl` helper function to the context in
which theme migrations are ran in. This helper function is to make it
easier for theme developers to check if a string is a valid URL or path
when writing theme migrations. This can be helpful in cases when
migrating a string based setting to `type: objects` which contain `type:
string` properties with URL validations enabled.
This commit also introduces the `UrlHelper.is_valid_url?` method
which actually checks that the URL string is of the valid format instead of
only checking if the URL string is parseable which is what `UrlHelper.relaxed_parse` does
and is not sufficient for our needs.
a6624af66e