fix(init): skip existing files instead of erroring (#20434)

### What
Skip writing files from the template if the files already exist in the
project directory.

### Why
When I run deno init in a directory that already has a main.ts, or one
of the other template files, I usually want to initialize a workspace
around a file I've started working in. A hard error in this case seems
counter productive. An informational message about what's being skipped
seems sufficient.

Close #20433

cli/tests/integration/init_tests.rs

@@ -128,3 +128,44 @@ fn init_subcommand_with_quiet_arg() {
   assert_contains!(output.stdout(), "1 passed");
   output.skip_output_check();
 }
+
+#[test]
+fn init_subcommand_with_existing_file() {
+  let context = TestContextBuilder::new().use_temp_cwd().build();
+  let cwd = context.temp_dir().path();
+
+  cwd
+    .join("main.ts")
+    .write("console.log('Log from main.ts that already exists');");
+
+  let output = context.new_command().args("init").split_output().run();
+
+  output.assert_exit_code(0);
+  output.assert_stderr_matches_text(
+    "ℹ️ Skipped creating main.ts as it already exists
+✅ Project initialized
+
+Run these commands to get started
+
+  # Run the program
+  deno run main.ts
+
+  # Run the program and watch for file changes
+  deno task dev
+
+  # Run the tests
+  deno test
+",
+  );
+
+  assert!(cwd.join("deno.json").exists());
+
+  let output = context
+    .new_command()
+    .env("NO_COLOR", "1")
+    .args("run main.ts")
+    .run();
+
+  output.assert_exit_code(0);
+  output.assert_matches_text("Log from main.ts that already exists\n");
+}

cli/tools/init/mod.rs

@@ -14,13 +14,22 @@ fn create_file(
   filename: &str,
   content: &str,
 ) -> Result<(), AnyError> {
-  let mut file = std::fs::OpenOptions::new()
-    .write(true)
-    .create_new(true)
-    .open(dir.join(filename))
-    .with_context(|| format!("Failed to create {filename} file"))?;
-  file.write_all(content.as_bytes())?;
-  Ok(())
+  let path = dir.join(filename);
+  if path.exists() {
+    info!(
+      "ℹ️ {}",
+      colors::gray(format!("Skipped creating {filename} as it already exists"))
+    );
+    Ok(())
+  } else {
+    let mut file = std::fs::OpenOptions::new()
+      .write(true)
+      .create_new(true)
+      .open(path)
+      .with_context(|| format!("Failed to create {filename} file"))?;
+    file.write_all(content.as_bytes())?;
+    Ok(())
+  }
 }
 
 pub async fn init_project(init_flags: InitFlags) -> Result<(), AnyError> {