markdown normalizer will _normalize_ mentions & references into persistent decentralized identifiers. ``` @alice.com -> [did:plc:alice] https://tangled.org/tangled.org/core/pulls/123 -> [at://...] ``` normalized content will be stored under `.text` in `sh.tangled.markup.markdown` type while orininal content will be kept under `.original`. This will allow understanding references while ingesting from third-party appviews Signed-off-by: Seongmin Lee <git@boltless.me>