Add tombstone rule as a default rule in support of MSC1930
Add tombstone rule as a default rule in support of MSC1930

Part of https://github.com/vector-im/riot-web/issues/8447

See also https://github.com/matrix-org/matrix-doc/pull/1930
matrix-org/matrix-js-sdk,matrix-org/matrix-js-sdk,matrix-org/matrix-js-sdk,matrix-org/matrix-js-sdk