fix(add): error when config file contains importMap field (#25115)

The "imports" field has higher precedence than "importMap", so we should
error when `deno add` goes to add an `"imports"` field.

Closes https://github.com/denoland/deno/issues/24264
Closes https://github.com/denoland/deno/pull/24478

cli/tools/registry/pm.rs

@@ -289,6 +289,17 @@ pub async fn add(
     _ => bail!("Failed updating config file due to no object."),
   };
 
+  if obj.get_string("importMap").is_some() {
+    bail!(
+      concat!(
+        "`deno add` is not supported when configuration file contains an \"importMap\" field. ",
+        "Inline the import map into the Deno configuration file.\n",
+        "    at {}",
+      ),
+      config_specifier
+    );
+  }
+
   let mut existing_imports = config_file.existing_imports()?;
 
   let is_npm = config_file.is_npm();

tests/specs/add/error_import_map_field/__test__.jsonc

@@ -0,0 +1,5 @@
+{
+  "args": "add jsr:@denotest/add",
+  "output": "add.out",
+  "exitCode": 1
+}

tests/specs/add/error_import_map_field/add.out

@@ -0,0 +1,2 @@
+error: `deno add` is not supported when configuration file contains an "importMap" field. Inline the import map into the Deno configuration file.
+    at file:///[WILDLINE]/deno.json

tests/specs/add/error_import_map_field/deno.json

@@ -0,0 +1,3 @@
+{
+  "importMap": "./import_map.json"
+}

tests/specs/add/error_import_map_field/import_map.json

@@ -0,0 +1 @@
+{}