Port/clean-up TextFlags encoders/decoders over to the new serialization format
<rdar://problem/100852252>
Pull request: https://github.com/WebKit/WebKit/pull/5084
Committed 255249@main (124e37f03891): <https://commits.webkit.org/255249@main> Reviewed commits have been landed. Closing PR #5084 and removing active labels.