templater: add Email template type, deprecate `Signature.username()` #17086

Job	Run time
Check protos	27s
Build jj-lib without Git support	51s
cargo-deny (advisories)	25s
build (ubuntu-latest)	5m 12s
Clippy check	2m 38s
cargo-deny (bans licenses sources)	25s
Check that MkDocs can build the docs	11s
build (macos-13)	6m 16s
build (macos-14)	3m 52s
Check that MkDocs can build the docs with latest Python and uv	16s
Check formatting	20s
build (windows-latest)	9m 54s
build (ubuntu-latest, --all-features)	6m 7s
	36m 54s

Provide feedback