Add websocketsharp.core to registry

[forkercat]

This PR adds the websocketsharp.core package to the registry.

The NuGet package needs to respect a few constraints in order to be listed in the curated list:

• Add a link to the NuGet package: https://www.nuget.org/packages/websocketsharp.core
• It must have non-preview versions (e.g 1.0.0 but not 1.0.0-preview.1) - version provided: 1.0.0
• It must provide .NETStandard2.0 assemblies as part of its package - version: >= .NETStandard2.0
• The lowest version added must be the lowest .NETStandard2.0 version available
• The package has been tested with the Unity editor - tested in my app in editor
• The package has been tested with a Unity standalone player - tested in a server build
  • if the package is not compatible with standalone player, please add a comment to a Known issues section to the top level readme.md
• All package dependencies with .NETStandard 2.0 target must be added to the PR - no dependencies
  • Note that if a future version of the package adds a new dependency, this dependency will have to be added manually as well

Note: The server will be updated only when a new version tag is pushed on the main branch, not necessarily after merging this pull-request.

[forkercat]

Hello @xoofx @bdovaz, greetings! This PR is ready for going through the workflow. Wanted to know if there are extra steps to get this change in. Thanks!

[forkercat]

The workflow passed. This PR is ready for a merge.

[forkercat]

Hello @bdovaz, thanks for merging this PR in!

I noticed that the websocket still didn't show up in Package Manager. Does it mean something go wrong or I need to wait a bit?

[bdovaz]

Already available in the new release!