Add default-run = "hx" to helix-term/Cargo.toml (#1244)

Following the addition of `xtask`, `cargo run` has multiple possible
targets, necessitating the usage of `cargo run --bin hx` to run Helix
during development. This allows `cargo run` to be used to run `hx`.

helix-term/Cargo.toml

@@ -9,6 +9,7 @@ categories = ["editor", "command-line-utilities"]
 repository = "https://github.com/helix-editor/helix"
 homepage = "https://helix-editor.com"
 include = ["src/**/*", "README.md"]
+default-run = "hx"
 
 [package.metadata.nix]
 build = true