Add snippet package manager

We added an experimental feature called `snippet` to mongosh. The product description in WRITING-8157 probably contains a good overview over what is provided here. This feature is experimental. Specifically, we do not offer support for it at this point, and provide it on a best-effort basis, and we may make changes to this feature or remove it at any time. https://github.com/mongodb-labs/mongosh-snippets contains a FAQ document with a few questions that we believe people might ask about this feature. More specifically, the following commands are provided: {code}   snippet install <name> Install a new snippet   snippet uninstall <name> Remove an installed snippet   snippet update Get the latest versions of installed snippets   snippet search List available snippets   snippet ls List installed snippets   snippet outdated List outdated snippets   snippet help Show this text   snippet help <name> Show information about a specific snippet   snippet info Show information about the snippet repository   snippet refresh Clear and refresh the snippet metadata cache   snippet load-all Load all installed snippets using load() {code}