Add DevToolsExtension template to devtools_extension package

[kenzieschmoll]

Work towards #1632.

[polina-c]

How about moving template out of lib to example and make it separate package with its own pubspec.yaml?
You will have to create example anyway, because it will be requested by pub.get publishing code.

[kenzieschmoll]

the template isn't an example - it is actually the file an extension author would import to write a devtools extension.

import 'package:devtools_extensions/api.dart';
import 'package:flutter/material.dart';

import 'package:devtools_extensions/template.dart';

void main() {
  runApp(const FooPackageDevToolsExtension());
}

class FooPackageDevToolsExtension extends StatelessWidget {
  const FooPackageDevToolsExtension({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Foo DevTools Extension',
      home: const DevToolsExtension(
        child: FooExtensionHomePage(),
      ),
    );
  }
}

[kenzieschmoll]

renamed to devtools_extensions.dart to make that more clear

[kenzieschmoll]

added an example dir in #6069