build: set `default-run` to `jj`
If not set, then `cargo run` produces this error:
```
error: `cargo run` could not determine which binary to run. Use the `--bin` option to specify a binary, or the `default-run` manifest key.
available binaries: fake-diff-editor, fake-editor, jj
```
@@ -12,6 +12,7 @@ documentation = "https://docs.rs/jujutsu"
readme = "README.md"
keywords = ["VCS", "DVCS", "SCM", "Git", "Mercurial"]
categories = ["command-line-utilities", "development-tools"]
+default-run = "jj"
[[bin]]
name = "jj"
