lexicons: add blobs field to `repo.{issue,pull}`
Ideally both `body` and `blobs` should be wrapped as single `body` field
with `sh.tangled.markup.markdown` type like `sh.tangled.feed.comment`
records, but that breaking change is deferred for later `org.tangled.*`
migration.
Signed-off-by: Wilhelm Berggren <wilhelmberggren@gmail.com>
Signed-off-by: Seongmin Lee <git@boltless.me>