Move notmuch json types into separate crate.

Add starter testdata and notmuch config.

notmuch/Cargo.toml

@@ -0,0 +1,9 @@
+[package]
+name = "notmuch"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+serde = { version = "1.0", features = ["derive"] }