Create and use a new logo for pub.dev

Currently, pub.dev uses the Dart logo and branding elements across the site. This works since it is about Dart packages, but it would be nice to see a separate logo for pub.dev, perhaps using the same or a similar color scheme.

This allows the site to have some distinctive branding as a separate but related product to the Dart language. One noticeable impact is the site favicon, which is used by browsers for tabs, allowing easier navigation to the site compared to dart.dev or other Dart sites. Some search engines also surface favicons for similar benefits. It would also allow us to link to the site with an icon from dart.dev and flutter.dev without it looking like a link to dart.dev.

There is some precedence for package repositories having separate logos/branding. Some are because they are ran/built by separate entities, but it's still common. Visit the links below for examples:

| Language/Ecosystem | Package repository |
| ------------- | ------------- |
| JS/[Node](https://nodejs.org/) | [npm](https://www.npmjs.com/) |
| [Rust](https://www.rust-lang.org/) | [crates.io](https://crates.io/) |
| [Python](https://www.python.org/) | [PyPi](https://pypi.org/) |
| [Ruby](https://www.ruby-lang.org/) | [RubyGems](https://rubygems.org/) |
| [C#](https://dotnet.microsoft.com/en-us/languages/csharp)/[.NET](https://dotnet.microsoft.com/) | [NuGet](https://www.nuget.org/) |
| [Fortran](https://fortran-lang.org/) | [fpm](https://fpm.fortran-lang.org/design/logo.html) |