Make new Connection("clusterName")work

[mikemaccana]

Motivation

Even Foundation and Labs people people call clusterApiURL() or have little helpers to match cluster names to connections. It would useful to allow Connection to just use a cluster name, if someone wants to connect to one of the official cluster URLs.

Example use case

Someone wants to connect to devnet.

They run:

const connection - new Connection("devnet");

And don't have to worry about matching the name to a network URL.

Details

Just check for a few magic values for the official clusters, anything outside those should be assumed to be a URL.

[steveluscher]

This runs up against the principle of greatest package efficiency. If you use the feature that exercises the clusterApiUrl logic, then you use some or all of that code. If you don't (ie. you supply your own RPC URL) then you use 0% of it, and waste those 628 gzipped bytes.

It's never a big deal in isolation, but if we allow this sort of pre-bundled-but-optional functionality everywhere (see #2115) then package efficiency goes way down.

[github-actions]

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