make --socket-path pull ETHERSYNC_SOCKET from env via clap

[cosmicexplorer]

Problem

It's much easier to set ETHERSYNC_SOCKET via the environment for many use cases instead of modifying the command line, as noted by @zormit in #112 (comment). We currently do this by hand in the nvim plugin:

ethersync/vim-plugin/plugin/ethersync.lua

Lines 69 to 72 in a41304a

	local socket_path = os.getenv("ETHERSYNC_SOCKET")
	if socket_path then
	table.insert(params, "--socket-path=" .. socket_path)
	end

We could do the same for the emacs plugin in #112, but it might be nice for the ethersync binary to support this itself.

Solution

• Add "env" feature to our clap dependency.
• Set Arg::env() for the global socket_path: PathBuf argument.
  • A command-line --socket-path argument overrides this value; the env var is only read if the argument is not provided.
  • Note that the daemon is now also affected by ETHERSYNC_SOCKET in addition to the client.
• Remove code in nvim plugin to translate the ETHERSYNC_SOCKET env var into --socket-path.

Result

All editor plugins (and the fuzzer) can expect to be able to set the ETHERSYNC_SOCKET environment variable as a separate way to parameterize the ethersync client, and users can set ETHERSYNC_SOCKET in their environment when invoking the daemon.

[blinry]

Thanks for the thoughtful PR! Very cool that clap has a built-in feature for this!

[blinry]

This seems really good to me! ✨

The last thing I'd request is undoing the mode change on fuzzer.rs spotted by @zormit :)

After that, it'd be great if you could squash the small "fixup" commits into the others, and then we can merge this! 🎉

I want to merge it now and "Changes requested" is blocking this :)

[zormit]

Thanks for this, @cosmicexplorer! =)