feat(add): always produce multiline config file (#23077)

This commit changes `deno add` to always produce a multiline config
file.

In v1.41.3:
```
$ mkdir foo
$ cd foo
$ deno add @std/assert
Created deno.json configuration file.
Add @std/assert - jsr:@std/assert@^0.220.0

$ cat deno.json
{ "imports": { "@std/assert": "jsr:@std/assert@^0.220.0" } }
```

Now:
```
$ mkdir foo
$ cd foo
$ deno add @std/assert
Created deno.json configuration file.
Add @std/assert - jsr:@std/assert@^0.220.0

$ cat deno.json
{ 
  "imports": {
    "@std/assert": "jsr:@std/assert@^0.220.0"
  }
}
```

2 files changed, 17 insertions, 6 deletions

diff --git a/cli/tools/registry/pm.rs b/cli/tools/registry/pm.rs
index 8a0a53c9a..a69a474e7 100644
--- a/cli/tools/registry/pm.rs
+++ b/cli/tools/registry/pm.rs
@@ -268,7 +268,15 @@ fn update_config_file_content(
       let insert_position = obj.range.end - 1;
       text_changes.push(TextChange {
         range: insert_position..insert_position,
-        new_text: format!("\"imports\": {{ {} }}", generated_imports),
+        // NOTE(bartlomieju): adding `\n` here to force the formatter to always
+        // produce a config file that is multline, like so:
+        // ```
+        // {
+        //   "imports": {
+        //     "<package_name>": "<registry>:<package_name>@<semver>"
+        //   }
+        // }
+        new_text: format!("\"imports\": {{\n {} }}", generated_imports),
       })
     }
     // we verified the shape of `imports` above
diff --git a/tests/integration/pm_tests.rs b/tests/integration/pm_tests.rs
index 668519bdc..a8af67e5b 100644
--- a/tests/integration/pm_tests.rs
+++ b/tests/integration/pm_tests.rs
@@ -39,11 +39,14 @@ fn add_basic_no_deno_json() {
   output.assert_exit_code(0);
   let output = output.combined_output();
   assert_contains!(output, "Add @denotest/add");
-  temp_dir.join("deno.json").assert_matches_json(json!({
-    "imports": {
-      "@denotest/add": "jsr:@denotest/add@^1.0.0"
-    }
-  }));
+  // Don't use `assert_matches_json` to ensure the file is properly formatted.
+  let expected = r#"{
+  "imports": {
+    "@denotest/add": "jsr:@denotest/add@^1.0.0"
+  }
+}
+"#;
+  temp_dir.join("deno.json").assert_matches_text(expected);
 }
 
 #[test]